Fighting between government troops and Russian-backed separatist rebels in a crucial railway hub threatens to scupper the cease-fire in eastern Ukraine, a day before both parties are due to start withdrawing heavy weaponry under a recently brokered deal.

Ukrainian officials say that the separatists in the country's east Sunday violated the cease-fire 111 times Sunday, resulting in the deaths of 5 soldiers and injuries to 25 more. On the other side, the rebels claimed 27 violations of the cease-fire by Kiev's forces. 

The most intense fighting remains centered around the key transport hub of Debaltseve, which connects the rebel-held cities of Donetsk and Luhansk. The rebels have surrounded approximately 8,000 Ukrainian troops inside the town and are bombarding them with a range of sophisticated Russian-made weaponry, including artillery, rockets and tanks. 

The separatists argue they should be granted immediate control over the town because they have it surrounded, and say that any force adopted against Ukrainian troops in Debaltseve would accordingly not violate the Minsk agreement. There are reports that ammo and food supplies in the town are running low. A rebel official told Fox News that the surrounded government troops will have to surrender and vowed to kill any soldier who tries to flee.

Observers from the Organization from Security and Cooperation in Europe (OSCE), who have been tasked with monitoring implementation of the peace deal, said Sunday that separatists have denied them access to Debaltseve. 

Associated Press reporters in Luhansk, about 9 miles to the northwest, heard sounds of sustained and regular shelling. Some of the artillery appeared to be outgoing, suggesting it was being fired by Ukrainian troops.

The fighting in Debaltseve has threatened a planned withdrawal of heavy weaponry from the front lines by both sides Tuesday. That plan already looks at risk with the rebels saying they are not satisfied the conditions are in place for the process to go ahead. Separatist military spokesman Eduard Basurin was quoted by as saying "We will begin pulling back equipment from the line of contact if we receive a certain signal, which is if the Ukrainians also do the same thing."

Basurin also said in a televised news conference on Monday that the government launched an artillery strike overnight on Horlivka, a town under rebel control. Ukraine, however, blamed the attack on the rebels. The government-appointed police chief of the Donetsk region, Vyacheslav Abroskin, said the separatists shelled the town in order to derail the truce.

In New York, UN Secretary General Ban Ki-moon expressed concern in a statement late on Sunday over continued hostilities around Debaltseve and reiterated his call "for all parties to abide by the cease-fire without exception."

The cease-fire, which was brokered last week in 16-hour talks involving the presidents of Russia, Ukraine, Germany and France, had raised cautious hopes for an end to the 10-month-old conflict, which has already claimed more than 5,300 lives.

Fox News' Greg Palkot and the Associated Press contributed to this report.