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.
Pope Francis on Wednesday said the threat of war in Ukraine had caused "great pain in my heart", and urged politicians to make a serious examination of conscience before God about their actions.
Ukraine's parliament on Wednesday approved imposing sanctions on 351 Russians, including lawmakers who supported the recognition of the independence of separatist-controlled territories and the use of Russian troops in eastern Ukraine.