Warning shots have been fired as a team of international military observers was turned back from entering Crimea.
It was the third time the OSCE has been prevented from entering Crimea, now in the control of pro-Russian forces.
In another development, Russia's deputy foreign minister has held talks with Ukraine's ambassador in Moscow.
The foreign ministry gave no details but said the talks on Saturday between deputy minister Grigory Karasin and ambassador Volodymyr Yelchenko were held in an "open atmosphere".
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At the scene
Ben BrownBBC News, inside a Ukrainian base in Crimea
The troops of Ukrainian air defence base A2355 are nervous, and who can blame them?
They've had to surrender all their weapons, and late at night pro-Russian forces attempted to ram the camp gates with an army lorry.
The precise identity of these armed men is unclear: are they Russian soldiers or simply, as Moscow insists, "self defence volunteers"?
Whoever they are, since they are armed and the Ukrainians inside the base are not, they could easily take it any time they want. All the Ukrainians have to stop them is a stinger contraption - a row of metal spikes to burst tyres.
The Ukrainians never know what will happen next. They look exhausted and mentally drained. As much as anything they seem to be the victims of a campaign of psychological intimidation.
Also on Saturday, US President Barack Obama and his French counterpart Francois Hollande warned of "new measures" against Moscow if it failed to make progress on defusing the crisis in Ukraine.
The French presidency said that, in a phone call, the two leaders insisted on the "need for Russia to withdraw forces sent to Crimea" and "to do everything to allow the deployment of international observers".
It was not clear what "new measures" could entail.
In other developments:
- Russian news agencies carried a statement from a defence ministry official saying that Moscow was considering halting foreign inspections of its strategic weapons arsenal - designated under international arms control treaties - as a result of US and Nato responses to the Ukraine crisis
- Poland's foreign minister said the country's consulate in Sevastopol had been "reluctantly" evacuated as a result of "continuing disturbances by Russian forces there"
Witnesses travelling with the OSCE said several shots were fired in the air as a convoy of vehicles approached a checkpoint manned by pro-Russian forces on a road leading from mainland Ukraine into the Crimea peninsula.
An OSCE spokeswoman said that the mission was withdrawing to the nearest big city, Kherson, to decide on its next steps.
The Vienna-based OSCE was invited by Ukraine's interim government, but Russian separatist authorities in Crimea say it does not have permission to enter the region.
In Simferopol, a ceremony was held for the swearing-in of the pro-Russia "Military Forces of the Autonomous Republic of Crimea"
Security was tight for a pro-Russia protest outside the regional parliament in Donetsk, east Ukraine
In Kiev, protesters voiced support for Crimea's Tatar population which feels increasingly threatened
Earlier, Russian Foreign Minister Sergei Lavrov said the crisis in Ukraine was "created artificially for purely geopolitical reasons".
He confirmed that Russia had contacts with Ukraine's interim government but said Kiev was beholden to the radical right.
Speaking to reporters in Moscow on Saturday, he said: "We are ready to continue a dialogue [with the West] on the understanding that a dialogue should be honest and partner-like, and without attempts to make us look like a party to the conflict. We didn't create this crisis."
Ukraine's crisis began in late November when President Viktor Yanukovych abandoned a landmark deal with the EU in favour of closer ties with Russia.
With no guns left, road spikes are the base's last line of defence, as Ben Brown reports
After weeks of protests in which more than 100 protesters were killed, Mr Yanukovych fled Ukraine and opposition leaders formed an interim government.
Russia effectively took control of Crimea - where its Black Sea Fleet is based - more than a week ago.
Crimea's pro-Russian leadership has set a date of 16 March for a referendum on joining Russia. Kiev says the vote would be illegal.
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Crisis timeline
- 21 Nov 2013: President Viktor Yanukovych abandons deal on closer ties with EU in favour of closer co-operation with Russia
- December 2013: Pro-EU protesters occupy Kiev city hall and Independence Square
- 20 Feb: At least 88 people killed in 48 hours of bloodshed in Kiev
- 21 Feb: President Yanukovych signs compromise deal with opposition leaders
- 22 Feb: President Yanukovych flees Kiev. Parliament votes to remove him and sets elections for 25 May
- 27-28 Feb: Pro-Russian gunmen seize key buildings in Crimean capital Simferopol
- 1 March: Russian parliament approves President Vladimir Putin's request to use Russian forces in Ukraine
- 6 March: Crimea's parliament asks to join Russia and sets referendum for 16 March
Dozens of military trucks transporting heavily armed soldiers arrived at a military airfield at Gvardeiskoe north of Simferopol on Saturday, AP news agency reported.
Licence plates and numbers indicated they were from the Moscow region, the report said, and some towed mobile kitchens and what appeared to be mobile medical equipment.
Russia denies sending troops to Crimea, but supports "local defence forces" protecting the largely ethnic-Russian population.
Also in Simferopol, a public ceremony was held for the swearing-in of the first unit in the pro-Russia "Military Forces of the Autonomous Republic of Crimea". Witnesses said about 30 armed men, ranging in age from teenagers to a man who appeared to be about 60, were sworn in at a city park.
Pro-Russian soldiers tried to seize a Ukrainian military base outside the biggest city, Sevastopol, overnight on Friday but no shots were fired and they pulled back.
The BBC's Ben Brown visited the base on Saturday and said that Ukrainian forces still inside were "nervous".
Western states have accused Russia of violating Ukrainian sovereignty in Crimea and both the US and EU have threatened Moscow with sanctions.