Pro-Russian soldiers are reported to have stormed a Ukrainian military base outside the Crimean city of Sevastopol, before withdrawing soon afterwards.
Two lorries from Russia's Black Sea Fleet were seen outside the gates, surrounded by armed men.
But no shots are believed to have been fired, and the assailants and lorries reportedly left after "negotiations".
Troops wearing Russian uniform without insignia have blockaded bases since taking control of Crimea last week.
Some military installations and other buildings in the peninsula have been taken over, but both sides have so far held their fire.
Earlier today, Russian President Vladimir Putin said he hopes there won't be a new Cold War over the crisis in Ukraine.
Vladimir Putin's spokesman says despite the deep differences with the West, he's optimistic common ground can be found.
The comments follow claims from Kiev that 30,000 Russian troops are now in Crimea. That's nearly twice the previous figure given by Moscow.
Russia was warned it could face further sanctions, unless it pulls its forces out of Ukraine. The French Foreign minister says if the first round of sanctions do not work the West will consider targeting businesses and individuals.
Overnight, US President Barack Obama spoke to Russian President Vladimir Putin on the phone for an hour, trying to convince him to accept the terms of a potential diplomatic solution to the Ukraine crisis.
After the call the Russian leader said the two sides were still far apart.
Both the EU and the US have denounced plans to have a referendum on whether Crimea should become part of Russia. It is understood the Russian parliament is in favour of a vote on on the issue.
Speaking at the European People's Party (EPP) congress in Dublin, Polish MEP Jacek Saryusz-Wolski said a vote is not legitimate.
Meanwhile Russia's parliamentary speaker has insisted there will not be a war with Ukraine. It comes as President Putin has again denied he is orchestrating events in the Crimean region.
Former Kremlin spin doctor Gleb Pavlovsky said there was now a greater danger of shots being fired in Crimea.
"Russia is encouraging the action of local forces. We are at a very dangerous point, and it threatens to push a political crisis in the direction of a military situation" he said.
The Ukrainian interim Prime Minister Arseny Yatseniuk says MPs had no right to announce it.
Russia now has more than 30,000 troops in Ukraine's disputed Crimea peninsular, according to reports.
Earlier Russian President Vladimir Putin denied he is "orchestrating events" in Crimea, and says he is simply responding to a request for help.
He spoke out after Moscow was warned it faces further sanctions if it fails to pull its forces out of Ukraine, as the gravest post-Cold War stand-off between the West and Russia continues.
Meanwhile, Ukraine's Paralympic chief Valeriy Sushkevich said his team would compete in the Winter Paralympics in the Russian Black Sea resort of Sochi.
But he said they would pull out of the Games, which begin this afternoon, if Russian forces invade mainland Ukraine.