NATO's top military commander has said Russia has all the forces it needs on the Ukrainian border if it was to decide to carry out an "incursion" into the country.
Russia could achieve its objective in three to five days, said US Air Force General Philip Breedlove.
Gen Breedlove, who is NATO's Supreme Allied Commander in Europe, called the situation on the border "incredibly concerning".
He said NATO had spotted signs of movement by a very small part of the Russian force overnight, but there was no indication that it was returning to barracks.
NATO foreign ministers meeting in Brussels had asked him to draw up by 15 April a package to reassure nervous NATO allies in eastern Europe that would include reinforcements by land, air and sea.
Meanwhile, Russia has accused NATO of using language that echoes the Cold War.
It said neither it nor NATO will gain from NATO's decision to suspend cooperation.
"The language of the statements rather resembles the verbal jousting of the Cold War era," said Foreign Ministry spokesman Alexander Lukashevich.
NATO Secretary General Anders Fogh Rasmussen said yesterday that NATO is "suspending all practical cooperation with Russia, military and civilian" over Russia's annexation of Crimea and the reported massing of troops near Ukraine's border.