SIMFEROPOL, Ukraine (AP) -- Dozens of heavily armed pro-Russia gunmen seized control of local government buildings in Ukraine's Crimea region early Thursday, escalating tensions in a peninsula that's become a flashpoint in the east-west divide that threatens to rip apart a country engulfed in political and economic turmoil.
Ukraine put its domestic security forces on high alert and urged Russian forces not to leave their base in southern Crimea. Failing to do so "will be considered a military aggression," Ukraine's acting president, Oleksandr Turchynov, said in the national parliament in Kiev.
The identity of the gunmen wasn't immediately clear and they had not put forth any demands Thursday morning, but witnesses said they were pro-Russian, wore unmarked camouflage uniforms, and carried rocket-propelled grenades, sniper rifles and other weapons. They raised the Russian flag over the local parliament building in Simferopol, the regional capital.
The renewed tension in this strategic peninsula that houses Russia's Black Sea fleet came as lawmakers in Kiev were expected to approve the new government that will face the hugely complicated task of restoring stability in a country that is not only deeply divided politically but on the verge of financial collapse.
After President Viktor Yanukovych's decision in November to reject an agreement that would strengthen ties with the EU and instead seek closer cooperation with Moscow protesters launched months-long protests that eventually forced him to flee last week after more than 80 people were killed in clashes between police and protesters. He has not been seen publicly since Saturday
A respected Russian news organization, meanwhile, said that Ynaukovych is staying in a Kremlin sanatorium just outside Moscow.
While the West has recognized the new Ukrainian government, whose forces drove Yanukovych from the capital, Russia still considers him the legitimate president.
Meanwhile, Russian news agencies quoted the Defense Ministry saying that as part of the military exercises in central and western Russia, fighter jets were put on combat alert. "Fighter jets are patrolling the air space in the border areas," the news agencies said quoting the military.
The ministry said 150,000 troops and 90 jets are involved in the maneuvers.
Russia has questioned the legitimacy of the new Ukrainian authorities after Yanukovych fled the capital last week, and it has accused them of failing to control radicals who threaten the Russia-speaking population in Ukraine's east and south, which includes the Crimean Peninsula.
The men occupying the local parliament building in Crimea did not immediately voice any demands but threw a flash grenade in response to a journalist's questions. They wore black and orange ribbons, a Russian symbol of the victory in World War II, and put up a sign saying "Crimea is Russia."
Maxim, a pro-Russian activist who refused to give his last name, said he and other activists who had camped out overnight outside the local parliament in Simferopol when heavily armed men wearing flak jackets and carrying rocket-propelled grenade launchers and sniper rifles took over the building.
"Our activists were sitting there all night calmly, building the barricades," he said. "At 5 o'clock unknown men turned up and went to the building. They got into the courtyard and put everyone on the ground.
"They were asking who we were. When we said we stand for the Russian language and Russia, they said: don't be afraid, we're with you. Then they began to storm the building bringing down the doors," he said.
"They didn't look like volunteers or amateurs, they were professionals. This was clearly a well-organized operation. They did not allow anyone to come near. They seized the building, drove out the police, there were about six police officers inside," he said.
"Who are they? Nobody knows. It's about 50-60 people, fully armed," he said.
In a statement, the local government said Crimean Prime Minister Anatoly Mogilyev had tried to negotiate with the gunmen but was told "they were not authorized to negotiate and present demands."
The events unfolding in Crimea highlighted the divided allegiances between Russia and the West that have deepened amid the political turmoil that has gripped Ukraine in recent months.
Russian President Vladimir Putin on Wednesday put the military on alert for massive exercises involving most of the military units in western Russia, and the military announced measures to tighten security at the headquarters of Russia's Black Sea Fleet on the peninsula.
The maneuvers will involve some 150,000 troops, 880 tanks, 90 aircraft and 80 navy ships, and are intended to "check the troops' readiness for action in crisis situations that threaten the nation's military security," Defense Minister Sergei Shoigu said in remarks carried by Russian news agencies.
The move prompted a sharp rebuke from U.S. Secretary of State John Kerry, who warned Russia against any military intervention in Ukraine.
Russia denied the military maneuvers were connected to the situation in Ukraine, but the massive show of force appeared intended to show both the new Ukrainian authorities and the West that the Kremlin was ready to use all means to protect its interests.
Ukraine's acting interior minister Arsen Avakov said on his Facebook page on Thursday that areas around the occupied buildings were being sealed off by police.
"Measures have been taken to counter extremist actions and not allow the situation to escalate into an armed confrontation in the center of the city," he said.
Phone calls to the Crimean legislature rang unanswered, and its website was down. Refat Chubarov, a local leader of the Tatar community that support the new authorities in Kiev, wrote on his Facebook page early Thursday that the two buildings were taken overnight by uniformed men.
The events in Crimea came as Ukraine's parliament was to form a new government after three months of street protests and violent clashes resulted in Yanukovych fleeing the capital.
On Wednesday, the interim leaders who seized control after Yanukovych disappeared proposed Arseniy Yatsenyuk as the country's new prime minister. The 39-year-old served as economy minister, foreign minister and parliamentary speaker before Yanukovych took office in 2010, and is widely viewed as a technocratic reformer who enjoys the support of the U.S.
Associated Press writers Maria Danilova and Karl Ritter in Kiev and Nataliya Vasilyeva in Moscow contributed to this report.