At least six Ukrainian soldiers were killed in renewed attacks on government military posts and checkpoints by separatists near the border with Russia, the Ukrainian military said on Tuesday.

There were also reports of an airstrike killing four civilians in the small town of Snizhne, for which Kiev denied responsibility and appeared to point a finger at Russia - two days after Moscow threatened retaliation for the death of man when a shell landed on the Russian side of the border.

"Today at 7 am, an unknown plane carried out a bombing attack on Snizhne. The flight can be described only as a cynical provocation," Andriy Lysenko, a spokesman for Ukraine's National Defense and Security Council, told reporters on Tuesday.

His remarks appeared to be an accusation directed at Russia, since the rebels have not used aircraft in the conflict.

Fighting over the past four months between forces of the Kiev government and separatists who want to join with Moscow has intensified since Friday, with Ukraine and Russia blaming each other for cross-border attacks and little prospect of a cease-fire.

Vladyslav Seleznyov, the spokesman for Kiev's "anti-terrorist operation" in the east, said rebel fighters had attacked military posts in several different areas overnight.

"As a result of a Grad missile attack during the night, two Ukrainian soldiers received fatal wounds," he told Fifth Channel television.

Security council spokesman Lysenko said six Ukrainian soldiers in total had died in the past day and that 13 were wounded.

Civilian deaths

Municipal officials said at least four civilians had been killed in Snizhne, 20 km from the Russian border, in what separatist rebels said had been an airstrike by a Ukrainian warplane.

A separatist spokesman, quoted by Russia's Interfax news agency, said 10 people had been killed.

Government officials, however, said that flights by Ukrainian warplanes had been suspended since Monday when a Ukrainian Antonov An-26 transport plane, carrying eight people, was downed in a rocket attack that Kiev said may have come from Russian territory.

Lysenko said two of the crew who survived appeared to have been taken captive.

Ukraine has said there is now clear evidence of direct Russian involvement in the fighting, which intensified over the weekend with Ukrainian airstrikes on rebel positions.

Well over 200 Ukrainian servicemen have been killed, as well as hundreds of civilians and rebels since violence erupted in Ukraine's Russian-speaking east following a pro-Europe revolt in Kiev that ousted a Moscow-backed president in February and led to Russia's annexation of Crimea.

Ukrainian President Petro Poroshenko, pressing the European Union to take a harder line with Moscow possibly involving new, tougher sanctions, said on Monday that Russian military officers were now fighting alongside separatists, and heavy military equipment was pouring across the border from Russia.

Poroshenko, elected in late May, refused on June 30 to extend a 10-day unilateral cease-fire, which he said had been breached repeatedly by the separatists. He renewed the government's military campaign to break rebel resistance.