The measure was among a number agreed on by US Secretary of State John Kerry and the other ministers, meeting at Nato headquarters in Brussels behind closed doors.

A civilian NATO official said the steps also included the possible deployment and reinforcement of military assets in eastern Nato members, such as Poland and the Baltic states, that feel menaced by Moscow's latest actions.

The alliance would also consider a possible increase of readiness levels for the Nato rapid response force and a possible review of Nato's crisis response plans, as well as its military training and exercise schedules.

NATO Supreme Commander General Phil Breedlove and his subordinates would draw up the proposals within a few weeks and then submit them to political leaders for their approval, the Nato official said.

Calling Russia's actions unacceptable, Nato chief Anders Fogh Rasmussen said: "Through its actions, Russia has undermined the principles on which our partnership is built, and has breached its own international commitments. So we cannot go on doing business as usual."

Russian President Vladimir Putin told Germany's Angela Merkel in a phone call on Monday that he had ordered a partial withdrawal of Russian troops from the eastern Ukrainian border, the German chancellor's spokesman said.

But Rasmussen said Nato saw no sign of this. "Unfortunately, I cannot confirm that Russia is withdrawing its troops. This is not what we are seeing," he said.

As Nato ministers met, Russia warned Ukraine against integration with Nato, saying Kiev's previous attempts to move closer to the defence alliance had had unwelcome consequences.

The US and its allies have made clear they have no military plans to defend Ukraine, which is not a Nato member, but they have assured allies in eastern Europe, which joined Nato in the last 15 years after the collapse of the Soviet Union, that they will be protected.

Rasmussen said Nato was "considering all options" to enhance its defences.

The United States and other Nato allies have already responded to the crisis by offering more planes to take part in regular Nato air patrols over the Baltic States. The United States has beefed up a previously planned training exercise with the Polish air force.

A Nato official said Russia had 35,000-40,000 troops stationed near the Ukraine border and that there was no sign of a significant reduction.

The Russian forces included mechanised infantry, armoured units, special forces and logistics units, the official said.

Ukraine crisis could be game-changer

As Moscow and the West dig in for a prolonged stand-off over Crimea, here are 10 ways in which the crisis could change attitudes and policy:

Moscow has been de facto excluded from the G8. Its bids to join the Organisation for Economic Co-operation and Development and the International Energy Agency are frozen.

As its mission in Afghanistan limps to a close, the US-led military alliance is back in business. An increase in allied air patrols and war games showing the flag in Poland and the Baltic states is on the agenda.

Europe's energy map is being redrawn with accelerated action to reduce dependence on Russian oil and gas. EU states will build more liquefied natural gas terminals, and expand gas supplies through Georgia and Turkey.

The diplomatic alliance between Russia and China could change in one of two directions - either rapprochement through a stronger energy partnership, with new pipelines being built to pump Russian oil and gas spurned by Europe to Beijing; or a cooling if China sees less benefit in closer ties with an economically weakened and relatively isolated Moscow.

Washington's global leadership role has been partially restored. Yet US strategists say American economic interests and the security challenges of managing a rising China mean Asia will remain the priority and Europe will have to do more for itself.

The Ukraine affair has cemented Berlin's leadership role in Europe. Chancellor Dr Angela Merkel has become Europe's main interlocutor with Vladimir Putin.

Greens European Parliament member Rebecca Harms joked that it was too early to nominate Putin for the annual Charlemagne prize for services to European unity, "but in the face of a new threat of war in Europe, EU states have indeed agreed on a joint strategy towards Russia".

Both Putin and the West are wooing central Asian autocrats in energy-rich Azerbaijan, Kazakhstan, Turkmenistan and Uzbekistan.

Moscow has an interest in avoiding greater isolation. But tensions are possible over Syria, Iran, Afghanistan or North Korea.

Russia's leader is near the peak of his popularity but instability may grow if he comes under pressure from magnates angry at losing value on their businesses.