Russia is dismissing reports it set a deadline for Ukranian forces in Crimea to surrender or face military action as 'complete nonsense'.
The Russian Black Sea fleet based in Crimea has denied it plans to storm Ukranian military positions on the peninsula.
It comes after earlier reports that Russia gave forces a deadline of 3am on Tuesday to surrender after troops seized key strategic sites in the region.
The Russian Prime Minister Dmitry Medvedev discussed the situation in Ukraine with US Vice President Joe Biden by telephone on Monday.
Mr. Medvedev "declared that it is necessary to protect the interests of all Ukrainian citizens, including residents of Crimea, and citizens of Russia who are located in Ukraine", according to Interfax.
He added that Russia would press ahead with plans to build a bridge linking Russia directly with the Crimea region providing a vital transport link to the Black Sea peninsula.
Mr. Medvedev told deputies the two countries had signed "documents related to a project for construction of a transport corridor across the Kerch Strait" in December, when now-ousted Ukrainian president Viktor Yanukovych was still in power.
The Crimea region in southern Ukraine
The Russian Foreign Ministry said NATO criticism of its actions in Crimea "will not help stabilise" the situation in Ukraine.
It said, "We believe that such a position will not help stabilise the situation in Ukraine and only encourages those forces that would like to use the current events to achieve their irresponsible political goals".
Hundreds of Russian soldiers have surrounded a military base in Crimea, preventing Ukrainian soldiers from going in or out.
The convoy blockading the site, near the Crimean capital Simferopol, includes at least 17 military vehicles.
Russian troops have also reportedly taken control of a ferry terminal in the city of Kerch, on the eastern tip of Crimea, which has a majority Russian-speaking population.
Ukraine's Defence Ministry said two Russian fighter jets violated the country's air space in the Black Sea on Sunday night and that it had scrambled an interceptor aircraft to prevent the "provocative actions".
Elsewhere, pro-Russian protesters have taken over a floor of the regional government building in Donetsk, say reports. The 11-storey building has been flying the Russian flag for the last three days.
Earlier, the Tánaiste and Foreign Affairs Minister said Russian actions over the weekend were "in effect, invading Ukraine". Eamon Gilmore was speaking at an Extraordinary Foreign Affairs Council in Brussels.
Mr Gilmore also said that their actions are in breach of United Nations charter and of international law.
He says the current situation in Crimea is putting Ukraine "on the brink of a very serious situation" and denied that the EU would stand by and do nothing:
Meanwhile Russia has said China is largely "in agreement" over the situation in Ukraine after the other G8 nations condemned its intrusion into the country.
Russian Foreign Minister Sergei Lavrov discussed Ukraine by telephone with his Chinese counterpart, Wang Yi, on Monday, and claimed they had "broadly coinciding points of view" on the situation there, according to a ministry statement.
