Poroshenko announced the week-long ceasefire on Friday night, urging the rebels to lay down their arms and warning that Ukrainian forces would hit back if attacked.
Meanwhile, Alexander Borodai, prime minister of the self-styled Donetsk People’s Republic said the ceasefire was not working and appealed to Russia to send in peacekeeping forces.
Russian President Vladimir Putin also ordered military forces in central Russia on combat alert as well as a drill of airborne troops.
The Kremlin had earlier dismissed Poroshenko’s plan, saying it sounded like an ultimatum and lacked any firm offer to open talks with insurgents. The combat alert in the central military district, which encompasses the Volga region and the Ural mountains but not western Russia, will last until next Saturday, Russian Defence Minister Sergei Shoigu said.
Later, Russian Foreign Minister Sergei Lavrov said Moscow is concerned that Ukraine is stepping up its military operation against pro-Russian rebels despite having called a unilateral ceasefire.
He made the remarks after meeting his Saudi counterpart for talks in which the war in Syria was expected to top the agenda.