Three Ukrainian servicemen were killed and 15 wounded in the past 24 hours in fighting with pro-Russian separatists, the Ukrainian military said on Tuesday - the first major casualties since a ceasefire came into force on Sept. 1.

Over the past two weeks monitors have reported a sharp decrease in hostilities along the eastern front line, fuelling hopes for the 18-month-old 'Minsk' peace process designed to end a conflict that has killed over 9,500 people since April 2014.

However, Ukrainian military spokesman Oleksandr Motuzyanyk said over the past day separatists had used heavy weapons for the first time this month and also directly attacked a Ukrainian position in Luhansk region.

"A sabotage-and-reconnaissance group crossed the river and attacked our servicemen. The fight lasted for 15 minutes," Motuzyanyk said in a daily televised briefing.

Two Ukrainian servicemen were killed in this encounter, while one was killed in shelling on the front line in Donetsk region, north-east of the Ukraine-controlled city of Mariupol. Another serviceman is missing in action, he said.

On Monday, separatist officials accused Ukraine of not honouring the truce, saying the Ukrainian military had fired heavy weapons at residential areas around the rebel-held Donetsk city, separatist news site DAN reported.

The so-called Minsk peace agreement, originally agreed between Russia, Ukraine, Germany and France in September, 2014, helped considerably reduce the fighting in east Ukraine's industrial Donbass region.

But violations have occurred regularly and implementing further points of the plan, including holding regional elections and returning control of Ukraine's border with Russia to Kiev, has long stalled.