SIMFEROPOL- Russian forces tightened their grip on Crimea on Sunday despite a U.S. warning to Moscow that annexing the southern Ukrainian region would close the door to diplomacy in a tense East-West standoff.

Russian forces' seizure of the Black Sea peninsula has been bloodless but tensions are mounting following the decision by pro-Russian groups that have taken over the regional parliament to make Crimea part of Russia.

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.

In the latest armed action, Russians took over a Ukrainian border post on the western edge of Crimea, trapping about 30 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.

In Simferopol, Crimea's main city, pro- and anti-Russian groups held rival rallies.
About 300 opponents of Russian-backed plans for Crimea to secede gathered around a monument to national hero Taras Shevchenko, carrying blue and yellow balloons - the colors 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, opposed Crimea joining Russia. "I don't call this a referendum. 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?"

Soviet Songs

Around 2,000 Russian supporters gathered in Lenin Square, where there is a statue of the Soviet state founder, clapping along to nostalgic Soviet-era songs being sung from the stage.

Alexander Liganov, 25 and jobless, said: "We have always been Russian, not Ukrainian. We support Putin."

President Vladimir 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.

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

"The main task is to preserve the stability and independence of our country," he said.

The worst face-off with Moscow since the Cold War has left the West scrambling for a response, especially since the region's pro-Russia leadership declared Crimea part of Russia last week and announced a March 16 referendum to confirm it.

U.S. Secretary of State John Kerry, speaking to Russia's foreign minister for the fourth day in a row, told Sergei Lavrov on Saturday that Russia should exercise restraint.

"He made clear that continued military escalation and provocation in Crimea or elsewhere in Ukraine, along with steps to annex Crimea to Russia, would close any available space for diplomacy, and he urged utmost restraint," a U.S. official said.

President Barack Obama spoke by phone on Saturday to the leaders of France, Britain and Italy, as well as three ex-Soviet Baltic states that have joined NATO. He assured Latvia, Lithuania and Estonia, which have their own ethnic Russian populations, that the Western military alliance would protect them if necessary.

Shots fired

A spokeswoman for the Organization for Security and Cooperation in Europe said military monitors from the pan-European watchdog had on Saturday been prevented for the third time in as many days from entering Crimea.

Shots were fired on Saturday to turn back the mission of more than 40 unarmed observers, who have been invited by Kiev but lack permission from Crimea's pro-Russian authorities to cross the isthmus to the peninsula. No one was hurt.

Crimea's pro-Moscow authorities have ordered all remaining Ukrainian troop detachments in the province to disarm and surrender, but at several locations they have refused to yield.

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.

A Reuters reporting team filmed a convoy of hundreds of Russian troops in about 50 trucks, accompanied by armoured vehicles and ambulances, which pulled into a military base north of Simferopol in broad daylight on Saturday.

The military standoff has remained bloodless, but troops on both sides spoke of increased agitation.

"The situation is changed. Tensions are much higher now. You have to go. You can't film here," said a Russian soldier carrying a heavy machine gun, his face covered except for his eyes, at a Ukrainian navy base in Novoozernoye.

A source in Ukraine's defense ministry said it was mobilizing some of its military hardware for a planned exercise, Interfax news agency reported. Ukraine's military, with barely 130,000 troops, would be no match for Russia's. So far, Kiev has held back from any action that might provoke a response.

Sanctions

Polish Foreign Minister Radoslaw Sikorski said on Saturday Poland had evacuated its consulate in Sevastopol due to "continuing disturbances by Russian forces."

The United States has announced sanctions against individuals it accuses of interfering with Ukrainian territorial integrity, although it has yet to publish the list. Washington has threatened wider action to isolate the Russian economy.

The European Union is also considering sanctions, but has so far been more cautious. Any action would be much harder to organise for a 28-nation bloc that takes decisions unanimously and many of whose members depend on Russian natural gas.

Pro-Moscow Crimea leader Sergei Aksyonov said the referendum on union with Russia - due in a week - would not be stopped. It had been called so quickly to avert "provocation," he said.

It is far from clear whether most of the 2 million Crimea residents want to be ruled by Moscow. When last asked in 1991, they voted narrowly for independence along with the rest of Ukraine.

Western countries dismiss the planned referendum as illegal and likely to be falsified.

Ukrainian PM to visit U.S.

Ukrainian Prime Minister Arseny Yatseniuk said on Sunday he would go to the United States this week to discuss the standoff with Russia over Ukraine's southern region of Crimea.

"I am going to the United States to hold top-level meetings on resolving the situation unfolding in our bilateral and multilateral relations," Yatseniuk said at the start of a government meeting in the Ukrainian capital, Kiev.

He did not immediately give any dates and provided no other details of the visit.

Kiev gov't freezes pro-Russia assets

Meanwhile, the new leadership in Kiev is freezing the bank accounts of members of the pro-Russian government in the Crimean peninsula.

The move makes it impossible for the autonomous region to conduct day-to-day business, Crimean Deputy Premier Rustam Temirgalyew said on Sunday in the regional capital Simferopol, adding that the region's leadership had already reached out to Russian banks to open new accounts.

The news comes amid further attacks on Ukrainian military bases in Crimea by suspected members of the Russian military.

According to Pentagon estimates there are currently 20,000 Russian soldiers active in Crimea. Moscow has thus far denied charges of deploying troops to the region.