Nato headquarters in Belgium have released a series of satellite images that appear to show almost 40,000 troops near the Ukrainian border.
The images, from satellite mapping company DigitalGlobe, show lines of tanks, artillery, helicopters and fighter jets that Nato claims could be deployed across the border in a "matter of hours."
Brigadier Gary Deakin, told reporters at Nato's Supreme Headquarters Allied Powers Europe that Russia's latest troop build up could have a "destablising effect" on the region and present "serious implications" for Ukraine's security.
However, an official in the Russian military general staff said on Thursday that the Nato photographs were taken during an exercise in August 2013, state-run news agency RIA reported.
Tensions remain high in eastern Ukraine after at least three died in a day of shooting during what was supposed to be an Easter truce.
In an exclusive interview with NBC News, the Ukrainian Prime Minister warns of the dangers of unchecked Russian expansion and aggression.
Russia, the US and the EU say that all sides have agreed to steps to "de-escalate" the crisis in Ukraine at talks in Geneva.