Ukrainian authorities on Saturday accused Russian troops of taking over numerous strategic targets in Crimea, an autonomous parliamentary republic within Ukraine, including the airport in the regional capital of Simferopol.
The soldiers also seized a border guard post in Sebastopol, home to Russia's Black Sea Fleet in Crimea, and a Ukrainian air force anti-missile base, those authorities said.
The head of the Ukrainian border guard service, Sergiy Astakhov, told local news agencies that the head of a column of 300 armed men told the Ukrainian border guards that they had received orders from the Russian Defense Ministry to take over a maritime guard post.
Earlier Saturday, Ukrainian Defense Minister Igor Tenyukh said Russia had sent 6,000 troops into the Autonomous Republic of Crimea in violation of bilateral accords.
"At this moment, Russia has increased its troops (in Crimea) by 6,000 soldiers ... and also mobilized 30 armored personnel carriers away from their regular bases in that region," Tenyukh said.
Russian soldiers are barring access to administrative buildings and military facilities, the minister said.
On Saturday, Ukraine's interim prime minister, Arseniy Yatsenyuk, demanded that Russian troops in Crimea pull back to their military installations, saying their deployment elsewhere in the peninsula violated bilateral accords.
Yatsenyuk issued the call as soldiers carrying Russian army machine guns took up positions next to the parliament building in Simferopol, Crimea's capital.
"The presence of Russian soldiers now in Crimea is inappropriate. It amounts to a violation of the basic provisions of the agreement governing ... the presence of Russia's Black Sea Fleet in Ukrainian territory," Yatsenyuk said.
"We are calling on the Russian government and authorities to order their troops to return to their bases," the interim prime minister, who took office on Thursday, said.
On Friday, U.S. President Barack Obama warned Russia about a military intervention in Crimea.
"We are now deeply concerned by reports of military movements taken by the Russian Federation inside Ukraine," Obama said, warning that there would be "costs" if a military intervention occurred.
Meanwhile, Russian President Vladimir Putin has reportedly asked senators' approval to use Russian troops stationed in Crimea to normalize the situation in that autonomous republic.
The Crimea Peninsula, located on the northern coast of the Black Sea, is home to 2 million inhabitants; ethnic Russians make up 60 percent of the population, while 25 percent are ethnic Ukrainians and 12 percent are Crimean Tatars.
The developments in Crimea pose an immediate challenge for Ukraine's new authorities, who were installed after the ouster of President Viktor Yanukovych following deadly protests last week.
The crisis that led to Yanukovych's ouster erupted at the end of November, when the Ukrainian president backed away from plans to ink a pact with the European Union and instead signed a $15 billion financial-aid package with Russia.
Protesters took to the streets of Kiev and began occupying administrative buildings, prompting Ukraine's parliament, then controlled by Yanukovych's allies, to pass a package of laws on Jan. 16 restricting freedom of assembly, freedom of speech and other basic rights.
Violent clashes pitting demonstrators and riot police broke out three days later and left six dead and hundreds wounded.
Ukrainian officials and opposition leaders began seeking a negotiated solution to the crisis in the wake of the bloodshed.
The talks led to the resignation of Prime Minister Mykola Azarov and the repeal of the controversial anti-protest laws.
Yanukovych departed Kiev after agreeing to call early presidential elections in a bid to put an end to bloody street battles last week in the capital that pit pro-European Union protesters against the security forces and left dozens dead.
After Yanukovych left the city, many of his former allies in parliament turned against him and helped oust him from power and release his arch-rival, the Fatherland's Yulia Tymoshenko, from prison.
She is a potential candidate in the presidential election, which has been moved up to May 25.
Yanukovych is believed to be in Russia, which has received a request from Ukraine for his extradition on mass murder charges. EFE