PEREVALNE, Ukraine (AP) — Igniting a tense stand-off, Russian forces surrounded a Ukrainian army base yesterday just as the country began mobilising its military in response to the surprise Russian takeover of Crimea. Outrage over Russia’s tactics mounted in world capitals, with US Secretary of State John Kerry calling on President Vladimir Putin to pull back from “an incredible act of aggression”.

Fearing that Europe’s borders were being rewritten by force, world leaders rushed to find a diplomatic solution to reverse what had already happened on the ground: Russia captured the Black Sea peninsula on Saturday without firing a shot.

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 yesterday in Kiev.

NATO held an emergency meeting in Brussels, Britain’s foreign minister flew to Kiev to support its new government and the US, France and Britain debated the possibility of boycotting the next Group of Eight economic summit, being held in June at 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 US Embassy in Kiev. One young girl held up a placard reading: “No Russian aggression!”

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

Kerry, interviewed Sunday on US 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. Kerry said all the foreign ministers he had talked to were prepared “to go to the hilt” to isolate Russia.

Still, politicians were treading carefully, knowing it was a delicate time for Europe.

“We are on a very dangerous track of increasing tensions,” 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 other countries have been powerless to counter the stealth Russian military tactics. Armed men in uniforms without insignia have moved freely about Crimea for days, occupying airports, smashing equipment at an airbase 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 between Russia and Europe. 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.