KIEV (Reuters) - Ukrainian troops have arrested two Russian servicemen in the country's separatist eastern territories, a Ukrainian military spokesman said on Sunday, reporting further casualties in rebel attacks despite a three-month-old ceasefire deal.

Ukraine and NATO accuse Moscow of supporting pro-Russian separatists with troops and military supplies, a charge the Kremlin has repeatedly denied.

"Two Russian servicemen are under arrest - our investigators are working with them," spokesman Andriy Lysenko said in a briefing, without giving further details.

Three Ukrainian servicemen were killed and 17 wounded in the past 24 hours, he said. Two of the deaths were a result of a mortar attack near the town of Svitlodarsk, northeast of separatist-controlled Donetsk.

While violence has lessened significantly since the ceasefire came into force in February, both sides accuse the other of violations and casualties are reported almost daily.

Senior rebel commander Eduard Basurin accused Ukrainian government troops of firing heavy weapons near Donetsk airport on Sunday, reporting that one rebel fighter had been killed and another wounded in the attack, rebel press service DAN said.

NATO's top commander, General Philip Breedlove, told the U.S. Congress last month that Russia's military might be using the truce to prepare for a new offensive in support of the separatists.