PEREVALNE, Ukraine >> Warning that it was “on the brink of disaster,” Ukraine put its military on high alert Sunday and appealed for international help to avoid what it feared was the possibility of a wider invasion by Russia.

Outrage over Russia’s military moves mounted in world capitals, with U.S. Secretary of State John Kerry calling on President Vladimir Putin to pull back from “an incredible act of aggression.”

A day after Russia captured the Crimean Peninsula without firing a shot, fears grew in the Ukrainian capital and beyond that Russia might seek to expand its control by seizing other parts of eastern Ukraine. Senior Obama administration officials said the U.S. now believes that Russia has complete operational control of Crimea, a pro-Russian area of the country, and has more than 6,000 forces in the region.

Ukrainian Prime Minister Arseniy Yatsenyuk said there was no reason for Russia to invade Ukraine and warned that “we are on the brink of disaster.”

“We believe that our western partners and the entire global community will support the territorial integrity and unity of Ukraine,” he said Sunday in Kiev.

World leaders rushed to try to find a diplomatic solution to the crisis.

NATO held an emergency meeting in Brussels, Britain’s foreign minister flew to Kiev to support its new government and Kerry was to travel to Ukraine Tuesday. The U.S., France and Britain debated the possibility of boycotting the next Group of Eight economic summit, to be held in June in Sochi, the host of Russia’s successful Winter Olympics.

In Kiev, Moscow and other cities, thousands of protesters took to the streets to either decry the Russian occupation or celebrate Crimea’s return to its former ruler.

“Support us, America!” a group of protesters chanted outside the U.S. Embassy in Kiev. One young girl held up a placard reading: “No Russian aggression!”

“Russia! Russia!” the crowd chanted in Moscow.

Kerry, interviewed on U.S. television news shows, talked about boycotting the G-8 summit, as well as possible visa bans, asset freezes and trade and investment penalties against Russia. All the foreign ministers he talked to were prepared “to go to the hilt” to isolate Russia, Kerry said.

President Barack Obama also spoke Sunday with German Chancellor Angela Merkel, British Prime Minister David Cameron, and Polish President Bronislaw Komorowski.

NATO issued a statement saying it “condemns Russia’s military escalation in Crimea” and demanding that Russia respect its obligations under the U.N. charter. Ukraine is not a NATO member, meaning the U.S. and Europe are not obligated to come to its defense, but the country has taken part in some alliance exercises.

“We are on a very dangerous track,” German Foreign Minister Frank-Walter Steinmeier said. But “it is still possible to turn around. A new division of Europe can still be prevented.”

So far, however, Ukraine’s new government and the West have been powerless to counter Russia’s tactics. Armed men in uniforms without insignia have moved freely about Crimea for days, occupying airports, smashing equipment at an air base and besieging a Ukrainian infantry base.

Putin has defied calls from the West to pull back his troops, insisting that Russia has a right to protect its interests and those of Russian-speakers in Crimea and elsewhere in Ukraine. His confidence is matched by the knowledge that Ukraine’s 46 million people have divided loyalties. While much of western Ukraine wants closer ties with the 28-nation European Union, its eastern and southern regions like Crimea look to Russia for support.

Russia has long wanted to reclaim the lush Crimean Peninsula, part of its territory until 1954. Russia’s Black Sea Fleet pays Ukraine millions every year to be stationed at the Crimean port of Sevastopol and nearly 60 percent of Crimea’s residents identify themselves as Russian.

During a phone conversation Sunday with Merkel, Putin “directed her attention to the unrelenting threat of violence from ultranationalist forces (in Ukraine) that endangered the life and legal interests of Russian citizens,” according to a Kremlin statement.

“The measures taken by Russia are fully adequate with regard to the current extraordinary situation,” it said.

Russia’s state-controlled media has played almost nonstop footage of the Ukrainian crisis, highlighting what it says are ultranationalist attacks on Russians and pro-Russian Ukrainians by activists from Kiev or regions further west. However, AP reporters in Ukraine witnessed no acts of violence directed at Russians or Russian sympathizers in Crimea.

Ukraine’s new government came to power last week following months of pro-democracy protests against the country’s pro-Russian president, Viktor Yanukovych, and his decision to turn Ukraine toward Russia instead of the EU. Yanukovych fled to Russia after more than 80 people were killed in the protests, but insists he’s still president.

Ukraine’s acting president, Oleksandr Turchynov, put Ukraine’s armed forces on alert Sunday, calling up reserves for training and stepping up security at nuclear power plants, airports and other strategic locations. However, no overt military actions by Ukraine were seen.

Turchynov also moved to consolidate the new government’s authority in eastern Ukraine, appointing 18 new regional governors and enlisting the support of the country’s wealthy businessmen, known as oligarchs. The new appointees included two oligarchs in the eastern cities of Dneprotrovsk and Donetsk, as big business and the Ukrainian government united against Russia.

Ukraine’s richest man, Rinat Akhmetov, urged business, ordinary people and the government to join together, saying Sunday that the use of force and “illegal action from outside” were “impermissible.”

“I call upon all my fellow citizens to unity for the sake of a whole and undivided Ukraine … Our strength is in the solidarity of business, government and society,” said Akhmetov, whose SCM Group has 300,000 employees and interests in steel, coal and mining.

Faced with the threat from Russia, “the national elite has consolidated around the new government,” political analyst Vadim Karasyov of the Institute for Global Strategies told The Associated Press. “This is a very good sign for the new government.”

Russian troops, meanwhile, pulled up to the Ukrainian military base at Perevalne on the Crimean Peninsula in a convoy Sunday that included at least 13 trucks and four armored vehicles with mounted machine guns. The trucks carried 30 soldiers each and had Russian license plates.

In response, a dozen Ukrainian soldiers, some with clips in their rifles, placed a tank at the base’s gate, leaving the two sides in a tense standoff. It appeared to be the first known case of outmatched Ukrainians standing up to Russian military might.

Unidentified soldiers were also seen cutting power to the headquarters of the Ukrainian naval forces in Crimea — whose commander defected later Sunday and pledged his allegiance to “the people of Crimea.”

In Kiev, a Ukrainian security official said the head of the Ukrainian navy — Adm. Denis Berezovsky — had been dismissed and faces a treason investigation after declaring his allegiance to the pro-Russian government in Crimea and offering no resistance to the Russian troops.

The speaker of Crimea’s legislature, Vladimir Konstantinov, was quoted as saying local authorities do not recognize the new government in Kiev. He said a planned referendum on March 30 would ask voters about the region’s future status.

A convoy of hundreds of Russian troops was also seen heading toward Simferopol, the regional capital of Crimea. Armed men in military uniforms without markings strolled around Simferopol’s central plaza, Lenin Square, outside its Council of Ministers building.

“It is very important that we all do everything we can to calm tensions,” said British Foreign Minister William Hague, who flew to Kiev on Sunday.

He said he has urged Russian officials to “speak directly to the Ukrainians” but so far they had not.

In Moscow, at least 10,000 people bearing Russian flags marched freely through the city Sunday, while a few dozen demonstrating on Red Square against the invasion of Ukraine were quickly detained by Russian riot police.

“We understand that the West wants to attack us and seize this territory. The West “is dangerous to us,” said Victor Sidelin, a Moscow resident at the march.

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McHugh reported from Kiev, Ukraine. Associated Press writers Lara Jakes and Julie Pace in Washington, Greg Keller in Paris, Laura Mills and Lynn Berry in Moscow, Greg Katz in London and Geir Moulson in Berlin contributed to this report.