NATO unveiled satellite photographs on Thursday it said showed Russian deployments of some 40,000 troops near the Ukrainian frontier, along with long lines of tanks, armored vehicles, artillery and aircraft ready for action.
"This is a force that is very capable, at high readiness, and, as we have illustrated through the imagery, is close to routes and lines of communication," British Brigadier Gary Deakin said at a briefing at NATO military headquarters at Mons in southern Belgium.
"It has the resources to be able to move quickly into Ukraine if it was ordered to do so," he said. If Russian political leaders took a decision to send forces into Ukraine, the first Russian forces could be on the move within 12 hours.
NATO has spotted Russian forces at more than 100 different sites close to the Ukraine border, he said.
The Western alliance displayed the commercial satellite images to reporters as evidence to back up its warnings of a Russian military buildup that could threaten Ukraine.
Russia denies massing forces near the border and accuses NATO of fomenting concern to rally support for the alliance.
Many of the sites in the images appeared to be fields rather than established military bases. The sites were located between about 40 kilometers (25 miles) and 150 kilometers of the Ukraine border, according to the images shown.
Russia's occupation and annexation of Ukraine's Crimea region has caused the biggest crisis in East-West relations since the Cold War and led to fears in Kiev that Moscow could send forces into other parts of Ukraine.
"They have all the capabilities: air, special forces, artillery. They have everything," Brigadier Deakin said.
The satellite pictures exhibited by NATO showed long lines of tanks, armored vehicles, artillery and aircraft parked and ready for action at a series of makeshift staging posts.
US Secretary of State John Kerry has accused Russian agents and special forces of stirring up separatist unrest and said Moscow could be trying to prepare for military action.
Russia denies the accusations and has dismissed concerns over a troop buildup.
In a buildup that began in early March, the satellite images showed that Russia has amassed not only infantry and artillery but electronic warfare units, helicopters and aircraft, said another NATO officer, speaking on condition of anonymity.
The pictures were taken between March 22 and April 2. NATO's assessment is that troop numbers have remained at about the same level since then.