Russia and the west are on a collision course over Crimea after Moscow was accused of orchestrating a "military invasion and occupation" of the peninsula, as groups of apparently pro-Russian armed men seized control of two airports. Russian troop movements were reported across the territory.

One Ukrainian official claimed late on Friday that 2,000 Russian troops had arrived in Crimea during the course of the day, in 13 Russian aircraft.

Ukraine's acting president, Oleksandr Turchynov, addressed the nation and accused Russia of carrying out a similar strategy to 2008, when it in effect annexed two Georgian territories, Abkhazia and South Ossetia. "They are trying to provoke a military conflict and are creating a scenario identical to the Abkhaz one, when having provoked a conflict, they annexed territory," he said.

Turchynov, installed following the removal of the pro-Moscow president Viktor Yanukovych over the weekend, appealed to Vladimir Putin to halt the incursion: "I am personally addressing President Putin to stop the provocation and call back the military from the Autonomous Republic of Crimea, and work exclusively within the framework of the signed agreements," he said.

Pro-Russian Cossacks rally outside the Crimean building in Simferopol. Ukraine has accused Russia of invading the peninsula. Photograph: Sean Gallup/Getty Images

On Friday evening the main Crimean air hub at Simferopol was still guarded by unidentified, uniformed men. Later it was announced that the airport had been closed and incoming flights diverted. There were similar scenes at Sevastopol airport. On Thursday pro-Russian gunmen seized the Crimean parliament in Simferopol.

"I see what has happened as a military invasion and occupation in violation of all international treaties and norms," said the new Ukrainian interior minister, Arsen Avakov earlier in the day. "This is a direct provocation aimed at armed bloodshed on the territory of a sovereign state."

Late on Friday Ukraine's defence ministry put out a statement saying it had information that unknown "radical forces" were planning to try to disarm its military units in Crimea early Saturday morning and warned against such action.

Armed men patrol at the airport in Simferopol, Crimea. Photograph: David Mdzinarishvili/Reuters

The White House said any Russian military intervention in Ukraine would be a "grave mistake", while the UN security council took up the issue at a session on Friday evening. A senior administration official said the US is considering pulling out of the G8 summit in Russia.

A US boycott of the June meeting would be a major blow to Putin, particularly if backed by European G8 members – the UK, Italy, Germany and France.

"We are consulting with European partners and considering options," the senior administration official told the Guardian. "It is hard to see how we and other European leaders would attend the G8 in Sochi if Russia is intervening in Ukraine."

Viktor Yanukovych gives a news conference in Rostov-on-Don, a city in southern Russia about 600 miles from Moscow. Photograph: Pavel Golovkin/AP

The sudden escalation of the crisis amounts to the most dangerous standoff in the former Soviet Union since the Russia-Georgia war six years ago.

As alarm grew during the day, Russia dismissed efforts by the new Ukrainian leadership to discuss the future of Crimea, a territory the size of Belgium which, despite a large Russian majority, has been part of Ukraine since independence two decades ago. Since 1991, Russia has maintained its own fleet at Sevastopol, a force that dwarfs Ukraine's own units in Crimea. The Russian foreign ministry said troop movements were "required to protect deployment places of the Black Sea fleet in Ukraine" and said the manoeuvres were fully in line with bilateral accords.

There was still uncertainty as to the precise identity of the gunmen holding the parliament and the airports. They claimed to be part of an informal self-defence group that has sprung up in response to the revolution in Kiev. But experts said they were hardly an impromptu militia.

"This is not a ragtag force," said Brigadier Ben Barry, a specialist on land warfare at the International Institute for Strategic Studies. "When you see a new militia, they will have a jumble-sale look. This lot are uniformly dressed and equipped and seem competent and efficient."

Russian nationalist politician Vladimir Zhirinovsky addresses a crowd in Sevastopol. Photograph: Baz Ratner/Reuters

Michael McFaul, until last week the US ambassador to Russia, wrote on Twitter: "If gunmen in Crimea are not acting on Kremlin's behalf, it would calming for Russian government to say so. Silence fuels uncertainty, instability."

Ukraine's national telephone operator said it had lost landline contact with Crimea.

The crisis was sparked by the bloody uprising in Kiev against the pro-Russian leadership that culminated in Yanukovych's flight last weekend. On Friday he resurfaced in the southern Russian city of Rostov-on-Don, denouncing the "bandit coup" in Kiev, and reiterating that he remained the legitimate president of Ukraine. In a floundering performance full of slip-ups and confused answers, Yanukovych called on Russia to act decisively, saying he was "surprised" by Putin's restraint.

Credit: Guardian graphics

He also said military action was unacceptable and the territorial integrity of Ukraine should not be violated. Yanukovych, who said he would not return to Ukraine until it was safe to do so, said presidential elections scheduled for 25 May were illegitimate.

There was an intense bout of international diplomacy over the increased tension, with David Cameron and German chancellor Angela Merkel speaking with Russian president Vladimir Putin. London said Putin and Cameron agreed to respect Ukraine's territorial integrity, while a Kremlin readout of the call merely said the leaders had agreed "there should be no further escalation of violence". The foreign secretary William Hague said he would be travelling to Kiev to meet the country's new leaders.

Political leaders moved fast in Moscow with the parliament rapidlyintroducing a law that would make it easier for new territories to be added to Russia's existing borders, a move that seemed directly linked to events in Crimea. The bill would allow for regions to join Russia by referendum if its host country does not have a "legitimate government". MP Elena Mizulina said: "If as the result of a referendum, Crimea appeals to Russia with a desire to join us, we should have the legal mechanisms to answer."

Russian nationalist politician Vladimir Zhirinovsky flew to Crimea and addressed cheering crowds in Sevastopol, promising them financial and psychological support against the new government in Kiev.

Another law under discussion would ease the requirements for Russian-speaking Ukrainians to receive Russian citizenship, and late on Friday, the Russian foreign ministry said it had ordered its consulate in Simferopol to begin "urgently" issuing passports to members of the Berkut riot police. The toughest regiments of police in Ukraine, Berkut regiments were used by Yanukovych against peaceful protesters. In the western city of Lviv, Berkut officers got down on their knees and begged forgiveness for the actions of their colleagues, but in Crimea, the returning troops have been greeted as heroes.

In Kiev, a new cabinet was voted in by the parliament on Thursday and needs to get to work to ease the appalling state of the economy, with Ukraine's currency weakening and the country facing a serious risk of default. The new government has been recognised as legitimate by most regions of Ukraine outside Crimea, but still has work to do to integrate law-enforcement bodies and restart the functioning of the state.

Ukraine's armed forces are dwarfed by Russia's – but would be no pushover if the Kremlin did decide to go for broke. "It is a nightmare for everyone," said Igor Sutyagin, a Russian military expert. "The entry of Russian troops would be a deep humiliation for Ukraine … It would be a second Chechnya."