Ukraine said on Thursday that pro-Russian rebels had reinforced their positions with tanks and missile systems in east Ukraine while marathon four-nation talks were taking place the previous night.
A deal reached at talks involving Ukraine, Russia, Germany and France envisages a ceasefire between Ukrainian forces and Russian-backed separatists starting on Sunday, but all sides said there was a long way to go towards real peace.
"The intensity of fighting is evidenced by a sharp increase in the number of people trying to leave front-line towns," Ukrainian military spokesman Andriy Lysenko said in a daily briefing a few hours after the agreement was reached in Belarus.
As the talks in Minsk dragged on through Wednesday night, around 50 tanks, 40 missile systems and 40 armored vehicles entered east Ukraine's breakaway Luhansk region from Russia via the Izvaryne border crossing, Lysenko said.
Russia denies supplying the rebels with arms and troops, despite what Ukraine and NATO say is overwhelming evidence to the contrary.
Separatists have strengthened their positions especially in Debaltseve, a strategic transport hub that has been the focus of heavy fighting in recent weeks, Lysenko said, stressing that government forces had not given up any ground.
But senior rebel commander Eduard Basurin said separatist forces had completely encircled Ukrainian troops in the town and their only option was to surrender. He said his side would honor the Minsk agreements.
"We are ready to organize the exit of Ukrainian forces from the Debaltseve encirclement on respectable conditions. They should put down their weapons and leave," Basurin told a news conference in Donetsk, broadcast live by Rossiya24 TV.
"Units of the army of Donetsk People's Republic are successfully holding their positions ... and are carrying out counter-attack operations to prevent the enemy from breaking from the encirclement."
He accused Ukrainian forces of attacking rebel positions in several places on Thursday and said there had been four cases of shelling of Donetsk in which two civilians were killed. The report could not be independently verified.
A Reuters witness in separatist-held Donetsk, south-west of Debaltseve, said artillery blasts could be heard sporadically throughout the day, although it was not clear in what direction the shells were being fired or by whom.
(Reporting by Alessandra Prentice in Kiev and Maxim Shemetov in Donetsk; Additional reporting by Gabriela Baczynska in Moscow; Editing by Mark Trevelyan)