Ukraine on Wednesday began calling up reservists into the armed forces and urged its citizens to leave Russia immediately as the pro-Western country braced for a possible invasion.
The call was issued a day after Russia's upper house of parliament gave President Vladimir Putin permission to send "peacekeepers" into two breakaway regions of eastern Ukraine.
Ukraine's armed forces said in a statement on Facebook that reservists between the ages of 18 and 60 would be called up for a period of up to one year.
Ukraine has a reserve force of more than 200,000 that is comprised mostly of military personnel who served in the first years of the country's eight-year war with Russian-backed separatists.
The decree signed by President Volodymyr Zelensky says former officers and non-commissioned personnel "will serve in military units and according to the specialization" that they had in the past.
"If the reservist, for one reason or another prescribed by law, cannot be called up, they must personally present an explanatory document," it added.
"Failure to appear or disregard for the call-up can lead to administrative and criminal penalties," it warned.
The foreign ministry said the growing threat of Russian "aggression" meant that it would be increasingly difficult to provide consular services for Ukrainian citizens in Russia.
Between two and three million Ukrainians are believed to be living in Russia today.
"The foreign ministry urges the citizens of Ukraine to refrain from travel to Russia, and for those in that country to leave its territory immediately," the ministry said.
Ukraine's Foreign Minister Dmytro Kuleba also reaffirmed his call for world powers to stiffen their sanctions on Russia.
Western powers began to announce new penalties on Tuesday directed mostly against Russian banks.
"To stop Putin from further aggression, we call on partners to impose more sanctions on Russia now," Kuleba said in a tweet.