Six Ukrainian soldiers have been killed and thirteen wounded in a fresh surge of fighting between pro-Russian rebels and government forces in the country's war-torn east, Kiev said Sunday.

Ukraine said Tuesday seven of its soldiers had been killed and 14 wounded in the bloodiest clashes with pro-Russian separatists in two months.

The announcement -- the highest daily death toll since Ukraine reported seven of its soldiers dying on May 24 -- follows a flurry of talks between world leaders and Moscow on stopping the 26-month conflict.

NATO chief Jens Stoltenberg on Wednesday demanded that Russia withdraw its forces and military hardware from Ukraine, and halt its support for pro-Moscow separatists battling Kiev.

European monitors observing Ukraine's flaring separatist conflict said on Wednesday that one of their local staff has gone missing in the pro-Russian east.

The Organization for Security and Cooperation in Europe (OSCE) said a member of its Special Monitoring Mission (SMM) did not return from a planned vacation on Tuesday and was reportedly being held in the insurgents' de facto capital of Donetsk.

Ukraine said Monday it had arrested a suspected far-right French extremist with a huge arsenal of weapons who allegedly planned to attack the European Championships in France this week.

The arrest added to jitters over the staging of European football's showcase event in France after twin attacks in Paris last year claimed by the Islamic State group killed 147.

Ukraine's pro-Russian eastern insurgents on Wednesday accused Kiev's soldiers of launching a new blitz near a prized but long-obliterated airport in the separatists' de facto capital of Donetsk.

The claim appears to fit with a mounting death toll reported by the pro-Western leadership in Kiev and foreign monitors from the Organization for Security and Co-operation in Europe (OSCE).

Seventeen people died when a makeshift home for elderly people outside the Ukrainian capital Kiev caught fire in the early hours of Sunday, the latest tragedy to shake the conflict-riven country.

The fire tore through the two-storey shelter for the elderly which is in the village of Litochky, located some 50 kilometers (31 miles) north of Kiev.

Ukrainian pilot Nadiya Savchenko, freed from a Russian jail this week, on Friday said she is ready to negotiate with pro-Russian separatists on releasing prisoners.

"I am ready to talk to the devil himself to get every one of our people back," she said at her first news conference since her dramatic release, part of a prisoner swap with Moscow that drew a line under a major diplomatic spat.

Ukrainian pilot Nadiya Savchenko, who was freed from jail in Russia as part of a prisoner exchange on Wednesday, has been condemned by Moscow as a murderer but is viewed by many in her Western-friendly country as a national hero.

The 35-year-old army helicopter navigator was handed a 22-year jail sentence in March over the killing of two Moscow state television journalists in the conflict in east Ukraine.

Ukrainian pilot Nadiya Savchenko has been freed after nearly two years in a Russian prison and is expected to return to a hero's welcome to Kiev shortly, defense laywers said Wednesday.

Savchenko's defense team confirmed Savchenko was on her way to Ukraine and thanked the pilot's supporters. "She is on her way home, to Ukraine," lawyer Mark Feigin said on Twitter.