Nato's supreme allied commander in Europe says Russia has massed all the forces it needs on Ukraine's border if it were to decide to carry out any "incursion" into the country.
US Air Force General Philip Breedlove said "we think it could accomplish its objectives in between three and five days if directed to make the actions".
North Atlantic Treaty Organisation (Nato) military chiefs are concerned that the Russian force on the Ukrainian border, which they estimate stands at 40,000 soldiers, could pose a threat to eastern and southern Ukraine.
Russian forces took control of Crimea, a strategic Ukrainian peninsula on the Black Sea, which then voted to unite with Russia in a March 16 referendum seen as a sham in the West. Moscow formally annexed Crimea on March 21.
Nato foreign ministers agreed on Tuesday (local time) to suspend all practical co-operation with Russia, draft measures to strengthen defences and reassure nervous Eastern European countries in the worst East-West crisis since the Cold War ended in 1991.
Moscow did not announce any measures to retaliate, but Foreign Minister Sergei Lavrov expressed concern over the moves in a phone call with US Secretary of State John Kerry, Russia's Foreign Ministry said.
"The language of the statements rather resembles the verbal jousting of the 'Cold War' era," Foreign Ministry spokesman Alexander Lukashevich said in a statement.
The suspension of Nato co-operation with Moscow means Russia cannot participate in joint exercises, although the alliance says joint work in Afghanistan on training counter-narcotics personnel, maintaining Afghan air force helicopters and providing a transit route out of the country could continue.
German Foreign Minister Frank-Walter Steinmeier said on Tuesday Nato's future relationship with Russia would depend, among other things, on whether Russia started withdrawing troops massed near Ukraine's eastern border.
"The statements about Moscow's allegedly aggressive intentions posing a threat to Nato member states are absolutely groundless," said Alexander Grushko, Russia's permanent representative at Nato, according to Interfax news agency.
Grushko noted that the last time Nato took such a decision, over Russia's five-day war with Georgia in 2008, the alliance later resumed co-operation of its own accord.
"It is not hard to imagine who will gain from the suspension of co-operation between Russia and Nato on countering modern threats and challenges to international and European security, in particular in areas such as the fight against terrorism, piracy and natural and man-made disasters," Lukashevich said.
"In any case, it will certainly not be Russia or Nato member states."
In Washington, US State Department spokeswoman Marie Harf said Kerry's phone call with Lavrov was brief and that Kerry "conveyed the strong support he was hearing for the people of Ukraine - and the legitimate government of Ukraine - from his counterparts during his Nato meeting in Brussels".
Harf also underscored US concerns about Russian troops along their shared border with Ukraine.