General Philip Breedlove, Supreme Allied Commander Europe, said Nato was concerned that the massed troops, tanks, helicopters and planes could also pose a threat to the separatist Transnistria region of Moldova.
The area broke away from Moldova in 1990 and has already sent a request asking to join the Russian Federation.
The Russian government said yesterday that its troops were in compliance with international agreements.
The warning comes after Russian troops, using armoured vehicles, automatic weapons and stun grenades, seized the last military facilities under Ukrainian control in Crimea, the Black Sea peninsular that Russian president Vladimir Putin officially annexed on Friday.
Gen Breedlove said yesterday: “The [Russian] force that is at the Ukrainian border now to the east is very, very sizeable and very, very ready.”
He said Nato was very concerned about the threat to Transnistria, which has not been recognised as an independent country by any United Nations member state.
About 30 per cent of its population of half a million is ethnic Russian, which is the mother tongue of an overall majority.
Russia launched a military exercise, involving 8,500 artillery men, near Ukraine’s border ten days ago.
Gen Breedlove added: “There is absolutely sufficient [Russian] force postioned on the eastern border of Ukraine to run to Transnistria if the decision was made to do that, and that is very worrisome.”
The president of Moldova warned Russia last Tuesday against considering any move to annex Transnistria, which lies on Ukraine’s western border, in the same way that it has taken control of Crimea.
The speaker of Transnistria’s parliament had urged Russia earlier to incorporate the region.
Russia’s deputy defence minister Anatoly Antonov was quoted by the state’s Itar-Tass news agency yesterday as saying that Russia was complying with international agreements limiting the number of troops near its border with Ukraine.
Moscow’s ambassador to the European Union, Vladimir Chizhov, also said that Russia did not have any “expansionist views”.
Asked to give a commitment that Russian troops would not move into other Ukrainian territory outside the Crimea, he said: “There is no intention of the Russian Federation to do anything like that.”
Meanwhile, the Belarussian president Alexander Lukashenko, a close ally of Russia, accepted yesterday that Crimea was now “de facto” a part of Russia, but he criticised the annexation as a “bad precedent”.
He said Ukraine, which shares a long land border with Belarus, should remain “a single, indivisible, integral, non-bloc state”.
Yesterday, the acting president of Ukraine called for the release of an air force commander held after his base in Crimea was stormed by pro-Russian forces.
Colonel Yuliy Mamchur is the commander of the Belbek Air Force base near the Crimean city of Sevastopol, which was taken over on Saturday by forces who sent armoured personnel carriers smashing through the base’s walls.
The Ukraine president Oleksandr Turchynov said Col Mamchur was “abducted” by the forces.
He did not specify where Col Mamchur is believed to be held. However, prominent politician Vitali Klitschko said he is being held by the Russian military in a jail in Sevastopol, the Crimean capital where Russia’s Black Sea Fleet has its base.
The Russian Defence Ministry said yesterday that the Russian flag was now flying over 189 military facilities in Crimea. It did not specify whether any Ukrainian military operations there remained under Ukrainian control.