KIEV, Ukraine — Ukrainian and Western leaders tried on Sunday to dissuade President Vladimir V. Putin of Russia from overplaying his hand and ordering an invasion of eastern Ukraine, even as Russian forces and their sympathizers in the Crimean Peninsula worked to disarm or neutralize any Ukrainian resistance there.

What began in Ukraine three months ago as a protest against the government of President Viktor F. Yanukovych has now turned into a big-power confrontation reminiscent of the Cold War and a significant challenge to international agreements on the sanctity of the borders of post-Soviet nations.

The Russian incursion also poses a new crisis for the Obama administration, which embraced the new government in Kiev but now finds itself confronted with an ever more thinly veiled invasion of Ukraine.

American intelligence agencies tracked thousands of additional Russian troops arriving in Crimea on Sunday, bolstering the Russian forces already in the area, an American official said. The official gave no further detail about the types of forces, and did not say whether the Obama administration believes that Mr. Putin will send even more troops in the days to come.

A senior Obama administration official said Russian troops now have “complete operational control of the Crimean Peninsula, with some 6,000 airborne and naval forces there. The official confirmed that the Russians were flying in additional reinforcements to Ukraine Sunday, adding that the Russian military is “settling in” as an occupying force.

For the most part, Ukrainian military forces have stayed in their barracks and in some cases their weapons have been stored in an attempt to avoid an escalation, the official said.

After the newly appointed Ukrainian Navy chief, Rear Adm. Denis Berezovsky swore allegiance to the people of Crimea, who are decidedly pro-Russian, an embarrassed Kiev immediately removed him and said it would investigate him for treason.

Secretary of State John Kerry will visit Kiev on Tuesday to show United States support for Ukraine, another senior United States official announced Sunday evening.

Chancellor Angela Merkel of Germany who spoke with Mr. Putin in a telephone call Sunday evening, accused Russia of violating the territorial integrity of Ukraine and breaking the Budapest Agreement of 1994 to respect the independence and sovereignty of Ukraine, according to a statement from Mrs. Merkel’s office.

Mr. Putin, the statement said, agreed to Ms. Merkel’s suggestion to send a “fact finding mission,” possibly led by the Vienna-based Organization for Security and Cooperation in Europe, to open a political dialogue.

The chancellor has maintained strong, if not always warm, ties with the Russian president and has often taken a leading role in Europe’s dialogue with Moscow. However, Germany, together with Poland, has also worked to bring Ukraine closer to the European Union.

The day began with Ukraine’s prime minister, Arseniy P. Yatsenyuk, telling reporters in English, “This is the red alert — this is not a threat, this is actually a declaration of war to my country,"a reference to approval by Russia’s Parliament on Saturday of the deployment of troops to any part of Ukraine where Moscow deems Russians to be in danger. Mr. Yatsenyuk warned that Ukraine was on the “brink of disaster” and asked the international community to stand by his government in Kiev.

Mr. Kerry on Sunday condemned Russia for what he called an “incredible act of aggression” and threatened “very serious repercussions.” He suggested what many were saying as fact later in the day, that Russia was “trying to annex Crimea.”

Britain, France and Germany joined the United States in suspending participation in preparatory meetings for the summit of the eight industrialized nations that Mr. Putin is to host in June in Sochi. Foreign Minister Laurent Fabius of France said on Europe 1 radio that Moscow must “realize that decisions have costs.” And Germany’s foreign minister, Frank-Walter Steinmeier, said that “we are on a very dangerous track of increasing tensions,” but that “it is still possible to turn around. A new division of Europe can still be prevented.”

In Moscow, Mr. Putin’s spokesman, Dmitri S. Peskov, responded dismissively. “It’s not a minus for Russia,” he said. “It will be a minus for the G-8.”

Britain’s foreign secretary, William Hague, traveled to Kiev on Sunday evening to meet with the new government and express support, and he said that was is urging restraint from all parties.

The NATO alliance held an emergency meeting in Brussels that was mostly designed to reassure members with Russian minorities, like the Baltics, and allies of Ukraine, like Poland, that NATO was ready to defend them. Ukraine is not a member of NATO, though it has some military and political cooperation with it.

Before the NATO meeting, its secretary-general, Anders Fogh Rasmussen, told Russia to stop its military activity and threats against Ukraine. “What Russia is doing now in Ukraine violates the principles of the United Nations Charter,” he said. “It threatens peace and security in Europe. Russia must stop its military activities and its threats.”

But it was difficult to see what immediate penalties would be put on Moscow to retreat on Crimea or to not broaden its military moves into eastern Ukraine. Mr. Putin seems to have decided that undermining the new, pro-European government in Kiev was worth most any plausible price in economic or diplomatic isolation, judging that the West would not react militarily.

In Moscow, there were some small protests of the military action, though they were quickly broken up by the police, even as many more demonstrated in favor of Mr. Putin’s actions.

Eastern Ukraine was relatively calm on Sunday, with the Ukrainian government making plans to reinforce its control by naming some prominent businessmen, with thousands of people dependent on them for work, as regional governors. Pro-Moscow demonstrators flew Russian flags on Saturday and Sunday at government buildings in cities including Kharkiv, Donetsk, Odessa and Dnipropetrovsk. In places, they clashed with anti-Russian protesters and guards defending the buildings.

In Crimea, where pro-Russian authorities have announced a referendum on autonomy on March 30, Ukrainian forces were under tremendous pressure. Hundreds of troops acting in the name of the provisional pro-Russian government in Crimea fanned out to persuade the thin Ukrainian forces there to give up their arms or swear allegiance to the new authorities, while the new government in Kiev tried to keep their loyalty while ordering them not to shoot unless under fire.

The former Ukrainian chief of staff, Adm. Yuriy Ilyin, who left the post on Friday after a reported heart attack, told reporters in Crimea that Ukrainian soldiers unfortunately were “hostages” of the situation. Emotions were made more complicated by the strong ties between the two navies, since the Ukrainian one was formed in the division of the Russian Black Sea Fleet.

In Crimea, fewer soldiers were visible on the streets. Some heavily armed soldiers without insignia had taken up positions around Ukrainian military bases, but did not try to enter them.

At Perevalnoe, about 15 miles south of Simferopol on the road to Yalta, hundreds of soldiers with masks, helmets and goggles, in unmarked uniforms, surrounded a Ukrainian marine and infantry base, using vehicles with Russian plates. Inside about two dozen Ukrainian soldiers could be seen, equipped with an old BMP armored personnel carrier.

The Ukrainian commander, Col. Sergei Starozhenko, 38, told reporters the unmarked troops had arrived about 5 a.m. and “they want to block the base.” He said he expected them to bring reinforcements and call for talks. Asked how many men he had at his command, he said simply, “Enough.” After 15 minutes of conversation with what appeared to be a Russian officer, he said, “There won’t be war,” and returned inside, while the standoff continued.

Pro-Russia demonstrators put up a banner reading: “Sevastopol without Fascism,” and urged Ukrainian officers to come over to their side rather than serve the “illegal fascist regime” in Kiev. The demonstrators pushed packs of cigarettes, candy and bottles of water through the gate for the Ukrainian guards.

Outside, Sergei Seryogin, a pro-Russia activist, said, “They have to make a choice — they either obey the fascists in Kiev or the people.” Kiev, he said, “is illegal power” and should be ignored by all military and civil officials.

At the Balaklava offices of the Ukrainian coast guard and border police, the Russian troop trucks that effectively besieged it on Saturday were gone. A member of the Sevastopol Council, Sergei Nepran, said that there had been an agreement with the Russians that the Ukrainians would remain in the office and not be put out to sea. Mr. Nepran claimed that the Sevastopol police “have come over to the people” and are now under control of a new pro-Russian mayor, Anatoly Chaly. Mr. Chaly, he said, had replaced a Kiev-appointed mayor who was forced to resign.

A Ukrainian Marine base in the Crimean port of Feodosiya was also surrounded, with the soldiers refusing to disarm. While Ukraine pulled its coast guard vessels out of Crimean ports, Kiev said its naval fleet’s 10 ships were still in Sevastopol and remained loyal.

On Sunday, Russia kept up its propaganda campaign in defense of the takeover, citing undefined threats to Russian citizens and proclaiming “massive defections” of Ukrainian forces in Crimea, which Western reporters said appeared to be unfounded. The state-owned Itar-Tass news agency cited the Russian border guard agency claiming that 675,000 Ukrainians had fled to Russia in January and February and that there were signs of a “humanitarian catastrophe.”

Russia insists that its intervention is only to protect its citizens and interests from chaos and disorder following the still unexplained departure from Kiev of the Kremlin-backed president, Mr. Yanukovych.

“If ‘revolutionary chaos’ in Ukraine continues, hundreds of thousands of refugees will flow into bordering Russian regions,” the border service said, according to Tass, providing another unsubstantiated justification for Russian military intervention.

In Kiev, Mr. Yatsenyuk, the prime minister, said he was “convinced” Russia would not intervene militarily in eastern Ukraine, “since this would be the beginning of war and the end of all relations between Ukraine and Russia.”