Despite the ceasefire brokered at last week's top-level Minsk summit, fighting continued on Monday in eastern Ukraine, The Associated Press reported.

Ukrainian government forces and Russian-backed separatists exchanged intense fire around the bitterly contested town of Debaltseve. The site of a strategically critical railway hub remains in Kiev's hands, but the rebels insist Kiev revert it to their control because they have encircled it.

"You can hear there is no ceasefire," an unnamed rebel fighter identified as "Scorpion" -- his nom de guerre -- told Reuters. "Debaltseve is our land. And we will take Debaltseve."

The United States accused the Russian armed forces of assisting the rebel operation around the town, a claim Moscow denied. Yuri Ushakov, a spokesman for Russian President Vladimir Putin, said the truce is "changing the situation dramatically" and that Moscow expects heavy weaponry to be pulled from the frontline.

Five Ukrainian troops were killed and 25 were wounded with the ceasefire technically in effect, Ukrainian military spokesman Andriy Lysenko said Monday. The violence, however, seemed to be focused on Debaltseve, and the deal seemed to hold elsewhere.

In Donetsk, the capital of the separatist Donetsk People's Republic, confrontations had subsided, and officials said there was no fighting in the area.

German Chancellor Angela Merkel, who in a joint initiative with French President François Hollande had brokered the deal at the Minsk meeting between Putin and Ukrainian leader Petro Poroshenko, admitted on Monday that the situation remained "fragile."

"It was always clear that much remains to be done. And I have always said that there are no guarantees that what we are trying to do succeeds," the chancellor said. "It will be an extremely difficult path."

Poroshenko accused Putin on Saturday of "significantly increasing" the Russian offensive in Ukrainian territory, the Independent noted. There had been reports of 40 tanks and almost 100 heavily armored vehicles and missile launchers crossing the Ukrainian-Russian border on Friday.

On Monday, the European Union reacted by adding 19 individuals and nine organizations to its list of sanctioned Russian entities. They include two Russian deputy defense ministers and several separatist commanders.