Germany's Angela Merkel delivered a rebuke to President Vladimir Putin on Sunday, telling him that a planned Moscow-backed referendum on whether Crimea should join Russia was illegal and violated Ukraine's constitution.

Putin defended breakaway moves by pro-Russian leaders in Crimea, where Russian forces tightened their grip on the Ukrainian Black Sea peninsula by seizing another border post and a military airfield.

As thousands staged rival rallies in Crimea, street violence flared in Sevastopol, when pro-Russian activists and Cossacks attacked a group of Ukrainians.

Chinese President Xi Jinping called for all parties to remain calm and urged a political solution to the crisis, during telephone calls with US President Barack Obama and Merkel.

"The situation in Ukraine is extremely complex, and what is most urgent is for all sides to remain calm and exercise restraint to avoid an escalation in tensions," China's foreign ministry on Monday cited Xi as telling Obama. "Political and diplomatic routes must be used to resolve the crisis," Xi added.

Russian forces' seizure of the region has been bloodless but tensions are mounting following the decision by pro-Russian groups there to make Crimea part of Russia.

In the latest armed action, pro-Russian forces wearing military uniforms bearing no designated markings sealed off a military airport in Crimea near the village of Saki, a Ukrainian Defence Ministry spokesman on the peninsula said.

The operation to seize Crimea began within days of Ukraine's pro-Russian President Viktor Yanukovich's flight from the country last month. Yanukovich was toppled after three months of demonstrations against a decision to spurn a free trade deal with the European Union for closer ties with Russia.

Ukrainian Prime Minister Arseny Yatseniuk will hold talks with President Barack Obama in Washington on Wednesday on how to find a peaceful resolution to the crisis, the White House said.

One of Obama's top national security officials said the US would not recognise the annexation of Crimea by Russia if residents vote to leave Ukraine in a referendum next week.

"We won't recognise it, nor will most of the world," deputy national security adviser Tony Blinken said.

INTERNATIONAL LAW

Putin declared a week ago that Russia had the right to invade Ukraine to protect Russian citizens, and his parliament has voted to change the law to make it easier to annex territory inhabited by Russian speakers.

Speaking by telephone to Merkel and British Prime Minister David Cameron, Putin said steps taken by authorities in Crimea were "based on international law and aimed at guaranteeing the legitimate interests of the peninsula's population," the Kremlin said.

A German government statement, however, said the referendum was illegal: "Holding it violates the Ukrainian constitution and international law." - Reuters