NATO said it was putting forces on standby and reinforcing eastern Europe with more ships and fighter jets, and could also send additional troops to its south-east flank, in what Russia denounced as an escalation of tensions over Ukraine.
Welcoming a series of deployments announced by alliance members in recent days, Secretary-General Jens Stoltenberg said NATO would take "all necessary measures".
The move was a further sign that the West is bracing for Russia to attack its neighbour after massing an estimated 100,000 troops in reach of the Ukrainian border.
"We will always respond to any deterioration of our security environment, including through strengthening our collective defence," Mr Stoltenberg said in a statement.
He later told a news conference that the enhanced presence on NATO's eastern flank could include the deployment of additional NATO battlegroups.
NATO Secretary General Jens Stoltenberg during a joint press conference. Source: EPA
"We are considering also to have battlegroups ... in the southeast of the alliance," he said.
So far, NATO has about 4,000 troops in multinational battalions in Estonia, Lithuania, Latvia and Poland, backed by tanks, air defences and intelligence and surveillance units.
Russia denies planning an invasion.
But, having engineered the crisis by surrounding Ukraine with forces from the north, east and south, Moscow is now citing the Western response as evidence to support its narrative that Russia is the target, not the instigator, of aggression.
US officials said the Pentagon was finalising efforts to identify specific units that it could deploy to NATO's eastern flank.
One of the officials said up to 5,000 could be deployed, while a NATO diplomat said Washington was considering gradually transferring some troops stationed in western Europe to eastern Europe in the coming weeks.
US President Joe Biden will hold a video call with European leaders later as part of the co-ordination with allies to respond to Russia's build-up on Ukraine's borders, the White House said.
Growing tensions
Denmark, Spain, France and the Netherlands were all planning or considering sending troops, planes or ships to eastern Europe, NATO said.
Ukraine shares borders with four NATO countries: Poland, Slovakia, Hungary and Romania. A Polish official said Warsaw would draw the line at sending troops to Ukraine.
In a sign of concern about the situation, Britain said it was withdrawing some staff and dependants from its embassy in Ukraine, a day after the United States said it was ordering diplomats' family members to leave. US diplomats are being allowed to leave voluntarily.
Kremlin spokesman Dmitry Peskov accused the West of "hysteria" and putting out information "laced with lies".
"As for specific actions, we see statements by the North Atlantic Alliance about reinforcement, pulling forces and resources to the eastern flank. All this leads to the fact that tensions are growing," he said.
"This is not happening because of what we, Russia, are doing. This is all happening because of what NATO and the US are doing and due to the information they are spreading."
Global stock markets skidded as the prospect of a Russian attack quashed demand for riskier assets such as bitcoin, and bolstered the dollar and oil.
The rouble hit a 14-month low against the dollar, and Russian stocks and bonds tumbled.
Russia has used its troop build-up to draw the West into discussions after presenting demands to redraw Europe's security map.
It wants NATO never to admit Ukraine and to pull back troops and weapons from former Communist countries in eastern Europe that joined it after the Cold War.
Washington says those demands are non-starters but is ready to discuss other ideas on arms control, missile deployments and confidence-building measures.
Russia is awaiting a written US response this week after talks last Friday - the fourth round this month - produced no breakthrough.
'Painful, violent and bloody'
Asked whether he thought an invasion was imminent, British Prime Minister Boris Johnson told broadcasters that intelligence was "pretty gloomy on this point" but that "sense can still prevail".
He repeated Western warnings that invading Ukraine would be "a painful, violent and bloody business" for Russia.
The United States and the European Union, wary of Russia's intentions since it seized Crimea and backed separatists fighting government forces in eastern Ukraine in 2014, have told Russia it will face crippling penalties if it attacks again.
EU foreign ministers meeting in Brussels warned Russia it would face "massive" consequences, but are divided over how tough to be on Moscow and did not say what the consequences might be.
Ukrainian President Volodymyr Zelenskiy told EU President Charles Michel by phone that it was important for Kyiv that the EU showed unity.
"Ukraine will not fall for provocations, and together with its partners, will remain calm and restrained," his office said.
