Thousands gathered at rival rallies in Ukraine as the White House announced Sunday that President Barack Obama will meet this week with Ukrainian PM Arseniy Yatseniuk (pictured) in a prominent show of support for the fledgling government.

By extending the invitation to Yatseniuk, the United States is sending a clear signal to Moscow that it considers Yatseniuk to be Ukraine's legitimate leader.

Yatseniuk was appointed interim premier by Ukraine's parliament after months of pro-Western demonstrations toppled pro-Russian president Viktor Yanukovich in February. Within days, Russian-backed forces had seized parts of Crimea.

The worst face-off with Moscow since the Cold War has left the West scrambling for a suitably robust response since the region’s pro-Russia leadership declared Crimea part of Russia last week.

In a latest round of diplomacy, German Chancellor Angela Merkel told President Vladimir Putin Sunday that the Moscow-backed plan to hold a referendum in Crimea on whether to rejoin Russia or remain a part of Ukraine was illegal and violated Ukraine’s constitution.

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

The German government, however, said the referendum was illegal. “Holding it violates the Ukrainian constitution and international law,” it said in a statement.

Putin has defended the breakaway moves by the region's pro-Russian leaders, and Russian forces tightened their grip on the Ukrainian Black Sea peninsula at the weekend by seizing another border post and a military airfield.

Putin declared last week that Russia had the right to invade Ukraine to protect Russian citizens. Russia's parliament also voted to change the law to make it easier to annex territory inhabited by Russian speakers.

US Secretary of State John Kerry, speaking to his Russian counterpart, Sergei Lavrov, for the fourth day in a row on Saturday, told him that Russia should exercise restraint.

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

“We won’t recognise it, nor will most of the world,” said the US deputy national security adviser, Tony Blinken.

Merkel said last week that if no progress was made in negotiations with Russia, the European Union could hit Russia with sanctions such as travel restrictions and asset freezes.

But Merkel, whose country is heavily dependent on Russia oil and gas, has so far been more cautious than some other nations, urging Western partners to give Putin more time before punishing Moscow with tough economic sanctions.

Not 'a single centimetre'

Ukraine's acting prime minister vowed on Sunday not to give up any of its territory to Russia.

“This is our land,” Yatsenyuk told a crowd gathered Sunday in the capital Kiev. “Our fathers and grandfathers have spilled their blood for this land. And we won’t budge a single centimetre from Ukrainian land. Let Russia and its president know this.”

In the afternoon, tens of thousands of Ukrainians massed in central Kiev for an interfaith prayer meeting to show their unity and honour the 200th anniversary of the birth of poet and nationalist Taras Shevchenko, the son of peasant serfs who is considered the father of modern Ukrainian literature.

One of the speakers, formerly imprisoned Russian tycoon Mikhail Khodorkovsky, almost burst into tears as he implored the crowd to believe that not all Russians support the country’s recent actions in Ukraine.

“I want you to know there is a completely different Russia,” Khodorkovsky said.

Russia seizes more border posts

The seizure of the region by pro-Russian forces has so far been bloodless but tensions are mounting. In the latest armed action, troops 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 Sunday.

Moscow denies that the Russian-speaking troops in Crimea are under its command, an assertion Washington dismisses as “Putin’s fiction”.

Although they wear no insignia, the troops drive vehicles with Russian military plates.

They took over a Ukrainian border post on the western edge of Crimea at around 6am (4am GMT), trapping about 15 personnel inside, a border guard spokesman said.

The spokesman, Oleh Slobodyan, said Russian forces now controlled 11 border guard posts across Crimea, a former Russian territory that is home to Russia’s Black Sea fleet and has an ethnic Russian majority.

Rival rallies

In Sevastopol, several hundred people held a meeting demanding that Crimea become part of Russia, chanting: “Moscow is our capital.”

Across town at a monument to the poet Shevchenko, violence flared as pro-Russian activists and Cossacks attacked a small group of Ukrainians at the event and police had to intervene.

Footage from the event showed a group of men violently kicking one of the Ukrainians as he lay on the ground and a Cossack repeatedly hit him with a long black leather whip.

In Simferopol, Crimea’s main city, pro- and anti-Russian groups held rival rallies.

Several hundred opponents of plans for Crimea to secede gathered, carrying blue and yellow balloons to match the colours of the Ukrainian flag. The crowd sang the national anthem, twice, and an Orthodox Priest led prayers and a hymn.

Vladimir Kirichenko, 58, an engineer, told Reuters he opposed the regional parliament’s plans for a vote this month on Crimea joining Russia.

“I don’t call this a referendum,” he said. “It asks two practically identical questions: Are you for the secession of Ukraine or are you for the secession of Ukraine? So why would I go and vote?”

Several thousand Russian supporters gathered in Lenin Square, clapping along to nostalgic Soviet-era songs.

Alexander Liganov, 25 and out of work, said: “We have always been Russian, not Ukrainian. We support Putin.”

At a rally in the eastern city of Donetsk, home to many Russian speakers, opposition leader presidential candidate Vitaly Klitschko, a former boxing champion, said Ukraine should not be allowed to split apart in bloodshed.

“The main task is to preserve the stability and independence of our country,” Klitschko said.

(FRANCE 24 with REUTERS and AP)