* Ukrainian minister says armed men are Russian forces
* Russian fleet denies involvement
* Swiss freeze assets of Yanukovich and son
* Kiev to seek Yanukovich extradition if he appears in Russia
* Ukrainian currency pull pulls of free fall
By Alissa de Carbonnel and Alessandra Prentice
SIMFEROPOL, Ukraine, Feb 28 (Reuters) - Armed men took control of two airports in the Crimea region on Friday in what Ukraine's government described as an invasion and occupation by Russian forces, stoking tension between Moscow and the West.
More than 10 Russian military helicopters also flew into Ukrainian airspace over the region on Friday, Kiev's border guard service said, accusing Russian servicemen of blockading one of its units in the port city of Sevastopol, where part of Moscow's Black Sea fleet is based.
However, the fleet denied its forces were involved in seizing one of the airports, Interfax news agency reported, while a supporter described the armed group at the other site merely as Crimean militiamen.
Tensions have been rising on the Black Sea peninsula, the only Ukrainian region that has an ethnic Russian majority and the last major bastion of resistance to the overthrow of Moscow-backed Viktor Yanukovich as president almost a week ago.
Moscow has promised to defend the interests of its citizens in Ukraine. While it has said it will not intervene by force, Russia's rhetoric since the removal of Yanukovich has echoed the run-up to its invasion of Georgia in 2008.
Ukraine's top security official, Andriy Paruby, said the armed men were taking their orders from the top in Russia. "These are separate groups ... commanded by the Kremlin," Paruby, secretary of the National Security and Defence Council, told a televised briefing in Kiev.
One of the options being considered was declaring a state of emergency in Crimea, he added.
The foreign ministers of France, Germany and Poland, who negotiated a peace deal to end violence in Kiev earlier this month, urged all parties to refrain from any action endangering Ukraine's territorial integrity.
Russia announced war games on Wednesday near the Ukrainian border, involving 150,000 troops on high alert, although U.S. Secretary of State John Kerry said his Russian counterpart, Sergi Lavrov, had told him the exercises were pre-planned.
Amid the confusion over the men's identity, acting president Oleksander Turchinov fired his armed forces chief, while parliament urged Moscow to halt any action that might encourage separatism and asked the United Nations Security Council to discuss the crisis.
Interior Minister Arsen Avakov accused Russian naval forces of taking over a military airport near the port of Sevastopol, where the Black Sea fleet has its base, and other Russian forces of seizing Simferopol's civilian international airport.
"I consider what has happened to be an armed invasion and occupation in violation of all international agreements and norms," Avakov said on his Facebook page, describing it as a "provocation" and calling for talks.
This met with a Russian naval denial of involvement in the military airport action. "No Black Sea Fleet units have moved toward (the airport), let alone taking any part in blockading it," Interfax quoted a spokesman for the fleet as saying.
Near the military airport, half a dozen men in camouflage uniforms with automatic rifles were blocking the road using a truck with no licence plates. Reporters were kept from approaching them by volunteer militia, who formed a second road block about 150 metres away.
"Of course they are Russian," said Maxim Lovinetsky, 23, one of the volunteers who manned the post. "They came last night."
Yanukovich is expected to appear before reporters in the Russian city of Rostov-on-Don later on Friday, though President Vladimir Putin has not said whether Moscow will harbour the former leader, who is on the run and wanted by the new government for mass murder after the deaths of protesters in Kiev last week.
In Kiev, the general prosecutor's office said on Friday that Ukraine would ask Moscow to extradite Yanukovich if it is confirmed that he is in Russia.
Switzerland, Austria and Liechtenstein moved on Friday to freeze assets and bank accounts of up to 20 Ukrainians including Yanukovich and his son.
Ukraine's new rulers have said loans worth $37 billion went missing from state accounts during Yanukovich's three years in power - a jaw-dropping sum even for a population now used to tales of his extravagance and lavish lifestyle, including his opulent residence outside Kiev.
The United States has told Russia to show in the next few days that it is sincere about a promise not to intervene in Ukraine, saying using force would be a grave mistake.
The Kremlin said Putin had ordered his government to continue talks with Ukraine on economic and trade relations and to consult foreign partners including the International Monetary Fund on financial aid.
Yanukovich provoked protests in Ukraine in November by backing out of plans to sign landmark deals with the European Union and instead saying Kiev would seek closer economic and trade ties with its former Soviet master Russia.
In December, Putin promised Yanukovich a $15 billion bailout, but Russia has put the deal on hold after releasing an initial instalment, saying it wants more clarity about the new government and its policies.
Ukraine's hryvnia rose on Friday from historic lows after the central bank governor limited access to foreign currencies. Dealers said the hryvnia was trading around 9.80-10.10 to the dollar after weakening as far as 11.20-10.10 on Thursday.
The hryvnia currency had been in freefall as investors worried about Kiev's ability to repay its debts.
Prime Minister Arseny Yatseniuk said Ukraine hoped to begin receiving international aid soon and was determined to fulfil conditions needed for IMF support. A previous deal collapsed after Kiev failed to implement IMF demands for lower gas subsidies, which would have hurt Ukrainians by pushing up energy prices sharply.
The IMF also wanted a more flexible currency regime, something that has now come about as Kiev gave up this week its attempts to arrest the hryvnia's slide, which had burnt through its dollar reserves.
Kiev's new rulers have said any movement by Russian forces beyond the base in Sevastopol would be tantamount to aggression. But they face a major challenge in Crimea which was Russian territory until it was transferred to Ukraine in 1954, during the Soviet era. Separatism there has often flared up at times of tension between Moscow and Kiev.
Unidentified gunmen seized the Crimean parliament and raised a Russian flag on Thursday. The gunmen issued no demands and police were casually guarding the building.
Armed men took control of Simferopol airport overnight and were patrolling its grounds on Friday morning.
A Reuters eyewitness at the scene said the men, dressed in full battle gear and carrying assault rifles and machine guns, were moving freely in and out of the control tower.
A man called Vladimir, who said he was a volunteer helping the group, said: "I'm with the People's Militia of Crimea. We're simple people, volunteers ... We're here at the airport to maintain order. We'll meet the planes with a nice smile - the airport is working as normal."
The regional parliament in Crimea managed to hold a session inside the building on Thursday despite the siege, where it voted to stage a referendum on "sovereignty" for Crimea.
Russia's flag still flew from its roof, and lights were on in the windows of its top floor. It was not clear whether the armed men were still inside.