Continued violence underscores the fragility of the peace accords, agreed in Minsk last month, which calls for the withdrawal of heavy weapons from the conflict's frontline. Some 6,000 people have been killed in fighting.
Interior Ministry adviser Zoryan Shkiryak said the rebels were not only not observing the peace agreement, but were in fact building up forces in the southern Donetsk province, near the Ukrainian-held port city of Mariupol.
"There is no use in talking about a ceasefire from the side of the rebels. They are not withdrawing arms, they are not observing the ceasefire, they continue to relocate units and increase their number of tanks and rocket launchers," Reuters quoted him as saying.
Another military spokesman said rebels had violated the ceasefire with 40 attacks on Ukrainian troops' positions and civilian targets, including 17 artillery attacks.
Earlier in February, after nearly 16 hours of negotiations in the Belarusian capital of Minsk, the leaders of Germany, France, Russia and Ukraine agreed a deal to end fighting in eastern Ukraine.
The new deal calls for a ceasefire between Ukrainian forces and Russian-backed separatists, the withdrawal of heavy weapons from the front line and constitutional reform to give eastern Ukraine more autonomy.