* Obama warns Russia over reports of military movements
* Obama, European leaders could skip G8 summit
* Ukraine accuses Russia, Moscow denies involvement
By Pavel Polityuk and Alessandra Prentice
KIEV/SIMFEROPOL, Ukraine, March 1 (Reuters) - Ukraine accused Russia on Saturday of sending thousands of additional troops to the southern Crimea region, which has a majority ethnic Russian population, and said it had placed its military in the area on high alert.
Russia said unidentified gunmen sent by Kiev had attempted overnight to seize the Crimea region's Interior Ministry offices and that people had been wounded in the attack. It accused Kiev of a "treacherous provocation".
Ukrainian Prime Minister Arseny Yatseniuk urged Moscow to cease what it called provocative actions, echoing a warning by U.S. President Barack Obama who said any military intervention following the overthrow of Russian-backed President Viktor Yanukovich would have costs for Moscow.
Armed men wearing combat uniform with no identification markings control two airports in Crimea, which hosts Russia's Black Sea Fleet, and have taken over the regional parliament in what Kiev described as an occupation by Moscow's forces.
"It is unacceptable when armoured Russian military vehicles are out in the centre of Ukrainian towns," Yatseniuk said at the start of a government meeting in Kiev.
"We do not give in to provocative actions, we do not use force and we demand that Russia stop its provocative actions and return the troops to base."
Russia says any movements by its military in Crimea are in line with agreements with Ukraine in the lease of the naval base in the port city of Sevastopol and accused Kiev of trying to destabilise the Black Sea peninsula.
Russia's Foreign Ministry said Kiev-backed gunmen had attempted to take over the offices of the Crimean Interior Ministry. It said people had been wounded but gave no details. There was no confirmation of such an action from other sources.
"With decisive actions by self-defense groups, the attempt to seize the interior ministry building was averted. This confirms the desire of prominent political circles in Kiev to destabilise the peninsula," it said in a statement.
Ukraine's acting president, Oleksander Turchinov, said on Friday that Russia was following a scenario like the one before it went to war with fellow former Soviet republic Georgia in 2008 over two breakaway regions. The regions are now fully beyond the control of Tbilisi.
Defence Minister Ihor Tenyukh told Saturday's government meeting that Russia had "recently" brought 6,000 additional personnel into Ukraine and that the Ukrainian military were on high alert in the Crimea region.
Several military transport planes and about 10 military helicopters had entered Ukrainian airspace on Friday without permission, he said.
The crisis, which began after Yanukovich triggered protests by spurning a political and trade deal with the European Union, is stoking tensions in a geopolitical battle between East and West that has echoes of the Cold War.
"We are now deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine," Obama told reporters in Washington.
"The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine."
Any violation of Ukraine's sovereignty and territorial integrity would be "deeply destabilising," he said.
Obama and European leaders would consider skipping a G8 summit this summer in the Russian city of Sochi if Moscow intervened militarily, a senior U.S. official said.
The G8 includes the world's seven leading industrial nations and Russia. Putin considers hosting such events as a way to show how far Russia has come since the Soviet Union's demise in 1991.
Washington's relations with Moscow are already cool because of differences over the conflict in Syria, Putin's record on human rights and Russia's decision to harbour former U.S. spy agency contractor Edward Snowden.
The removal of Yanukovich from power has been accepted across Ukraine, even - grudgingly - in the eastern, mainly Russian-speaking regions that were his powerbase. But the new Ukrainian leader, Turchinov, faces a challenge in Crimea, the only region in the country that has an ethnic Russian majority.
Crimea was a Russian territory in the communist Soviet Union before Soviet leader Nikita Khrushchev gifted it to Soviet Ukraine in 1954. Some ethnic Russians want Russia to reclaim it now.
In a sign of defiance, Sergei Aksyonov, the pro-Russia prime minister of Crimea, has put himself in charge of all military forces, police and other security services in the region.
He has appealed to Putin for "assistance in guaranteeing peace and calm" there and Interfax news agency quoted a Kremlin source as saying the appeal would be considered by Moscow.
Gunmen took over the regional parliament in Crimea on Thursday, and have controlled the main international airport in Simferopol, the main regional centre, and a military airfield near Sevastopol since Friday.
A representative of Acting President Turchinov said 13 Russian aircraft had landed with 150 personnel on each plane.
A local television station reported that another military aerodrome had been taken over by armed men overnight, but the report was not independently confirmed.
Phone lines have been severed in some areas and witnesses say they have seen armoured personnel carriers on the move.
There has been no bloodshed and no military clashes since Yanukovich's flight from Kiev last week although Ukraine's leaders say about 100 were killed, some by police snipers, during protests in Kiev that began last November.
Yanukovich, 63, resurfaced in southern Russia on Friday after a week on the run, defiantly telling a packed room of journalists that he was still leader of the sprawling former Soviet republic of 46 million.
"Russia cannot be indifferent, cannot be a bystander watching the fate of as close a partner as Ukraine," Yanukovich told a news conference. "Russia must use all means at its disposal to end the chaos and terror gripping Ukraine."
Putin has said nothing in public about the crisis since Yanukovich was ousted a week ago.
A Kremlin statement this week offered conciliatory remarks about international cooperation over heavily indebted Ukraine but Russian officials have blamed the crisis on the West and accused it of meddling in what Moscow considers its back yard. Loss of influence in Ukraine is a bitter blow for Putin.
Moves are under way, however, to prop up Ukraine's economy. The new Ukrainian leadership has said the country needs about $35 billion over the next two years to stave off bankruptcy.
It said on Friday it hoped to get financial aid soon and was prepared to fulfil the reform criteria of the International Monetary Fund, which will visit Kiev next week.
The fate of a $15-billion Russian bailout package is unclear, with only about $3 billion of it released so far.