Poland has asked Nato to station 10,000 troops on its territory as a visible demonstration of the alliance's resolve to defend all its members after Russia's seizure of Crimea.
Nato foreign ministers met in Brussels to consider requests for soldiers to be deployed in Poland and the Baltic states, all of which share borders with Russia.
Nato generals and admirals have been told to improve protection of members that feel threatened by Russia, and to suspend "all practical civilian and military co-operation" with Moscow.
President Vladimir Putin has massed about 40,000 troops near Ukraine's eastern frontier, giving himself the option of seizing more of its territory. Yesterday, he assured German Chancellor Angela Merkel some of these forces would be withdrawn. But the Nato ministers said Russia had yet to keep that promise.
"We've had some statements from Russia about pulling back forces from the eastern border, but we haven't seen the evidence yet," said William Hague, the British Foreign Secretary.
Anders Fogh Rasmussen, the Nato Secretary-General, said he could not "confirm that Russia is withdrawing its troops" and warned of the dangers posed by a "massive military build-up".
Against this background, Radek Sikorski, the Polish Foreign Minister, asked for "two heavy brigades" of armoured infantry, with 5000 troops each, to be stationed in his country. Poland has a 230km border with Russia's Kaliningrad enclave.
"It's very important all members enjoy the same level of security," said Sikorski. "Poland has been a member of Nato for 15 years now, and so far the only permanent military institution we have is a conference centre, training facility. We would welcome a prominent, major presence."
The US may send another warship to the Black Sea and take other steps to reassure anxious allies in Eastern Europe after Russia's intervention in Ukraine.
An American destroyer was deployed to the Black Sea earlier in the crisis over Ukraine but departed last month.
Commanders also were looking at expanding a scheduled exercise in Europe this northern summer involving troops from the US Army's 1st Cavalry division, said a senior defence official travelling with Pentagon chief Chuck Hagel.
Britain has offered to send RAF Typhoon fighters for Nato exercises over Poland and air defence patrols above the Baltic states. But Poland wants permanent Nato forces on its territory.
Hague gave a cautious response, saying: "We certainly need to give additional reassurance and confidence to our eastern allies. The UK has said we will contribute aircraft to Baltic air policing and there may be other measures."
Nato is expected to make further announcements on the deployment of military assets in coming weeks, which may include sending troops and warships to eastern Europe and the Baltic.
However, Germany and other Nato members are wary of causing more tension with Russia by sending forces to its frontiers. "We don't need any Nato troops on the border with Russia," said Frans Timmermans, the Dutch Foreign Minister.