Ukrainian officials say Russia-back rebels are continuing their onslaught on the railroad junction of Debaltseve, the epicenter of fighting between the separatist and government troops in eastern Ukraine.

Fierce fighting around the town linking two major separatist cities rages on despite a cease-fire which went into effect on Sunday.

Ukrainian military Anatoliy Stelmakh said in a televised briefing on Wednesday that the rebels launched five artillery strikes on Debaltseve overnight.

Hundreds if not thousands of Ukrainian troops are believe to be trapped in Debaltseve, surrounded by advancing rebel troops.

Associated Press reporters were turned back a checkpoint outside Vuhlehirsk, about 10 kilometers (six miles) west of Debaltseve early on Wednesday and were unable to verify the situation there. Regular artillery fire was heard in the vicinity.