Ukrainian government troops and Russia-backed rebels failed Tuesday to start pulling back heavy weaponry from the front line in eastern Ukraine as a deadline passed to do so.

Under a cease-fire agreement negotiated by the leaders of Ukraine, Russia, Germany and France last week, the warring sides were to begin withdrawing heavy weapons from the front line on Tuesday. Both sides indicated Monday, however, that they would begin the pullout only after the other party does so.

Ukrainian military spokesman Anatoliy Stelmakh said in televised comments early on Tuesday that the separatists continued to attack their positions overnight and that the pullout hinges on the cease-fire being fully observed.

"As soon as the militants cease fire, the Ukrainian side will begin to withdraw heavy weaponry from the frontline," he said.

The Russian news agency RIA Novosti quoted rebel leader Andrei Purgin as saying that the separatists plan to discuss the possible withdrawal of the weaponry later Tuesday with representatives of Ukraine, Russia and the group charged with monitoring the ceasefire, the Organization for Security and Cooperation in Europe.

Kiev on Tuesday reported artillery strikes overnight around the key transportation hub of Debaltseve while the situation in the rest of the war zone appeared to be calm.

An Associated Press reporter heard sustained shelling around Debaltseve on Tuesday morning, some coming from Grad rocket launchers.

The leaders of Ukraine, Russia and Germany discussed the cease-fire in a phone call late Monday. Chancellor Angela Merkel's office reported that the three of them agreed on "concrete steps to enable an observation" of the situation in Debaltseve by the OSCE. OSCE representatives, however, did not manage to get to Debaltseve on Monday because of heavy fighting there.

Merkel and Ukrainian President Poroshenko appealed to Russian President Vladimir Putin to use his influence on the separatists to ensure that they stop fighting.