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. A senior U.S. official said Washington now believes that Russia has complete operational control of Crimea, a pro-Russian area of the country, and has more than 6,000 air, naval and ground 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 handful of protesters chanted outside the U.S. Embassy in Kiev. One young girl held up a placard reading: "No Russian aggression!"