KIEV (Reuters) - One Ukrainian serviceman was killed and five wounded in clashes between government forces and Russian-backed separatists over the past 24 hours despite a ceasefire, Ukrainian military spokesman Andriy Lysenko said on Saturday.

Violence has lessened significantly in Ukraine's east since the ceasefire was brokered in Minsk, Belarus, in mid-February, but both sides accuse the other of violations and casualties are reported almost daily.

The situation was most tense near the rebel-controlled city of Donetsk, where separatists were using heavy weapons banned under the ceasefire, Lysenko said. "Pro-Russian illegal groups are firing heavily there," he said.

Russia dismisses accusations from Ukraine, NATO and Western powers that is arming the separatists and supporting them with its own troops.

Separatists accused government forces of bringing heavy weapons, including tanks, artillery and fighter planes, close to the frontline in preparation for an offensive, rebel press service DAN reported.

This "clearly indicates Kiev's unwillingness to observe its commitments to the Minsk agreements and also that it's stepping up preparations for military action," senior rebel commander Eduard Basurin was quoted as saying in a briefing.

Separatist militias are "strictly adhering" to the ceasefire terms, but are "ready for decisive action to protect the republic," he said, referring to the self-proclaimed Donetsk People's Republic.

On Thursday, the Organization for Security and Cooperation in Europe expressed concern that violence appeared to be spreading in eastern territories.

It also accused both sides of moving weapons toward the frontline that had previously been withdrawn as required under the Minsk ceasefire deal.

(Reporting by Pavel Polityuk; Writing by Alessandra Prentice; Editing by Andrew Heavens)