Russia put more than 65,000 troops on combat alert and ordered them to take part in a drill a day after Ukraine called a week-long cease-fire to quell violence in the eastern part of the country.
The Russian drill is the biggest since the country annexed the Black Sea Crimean peninsula in March. The U.S. has accused the government in Moscow of aiding the separatists and this week imposed sanctions on people linked to the insurgency.
The dispute is flaring as American and European officials warn that more painful penalties affecting Russiaâ€™s access to financial markets, technology and military hardware may come as early as next week if President Vladimir Putin refuses to curb tensions. North Atlantic Treaty Organization and U.S. officials have said this week that Russia was renewing its military buildup near the Ukrainian border.