Three Ukrainian soldiers were killed and seven injured during a new flareup in clashes between government forces and pro-Russian eastern separatist rebels, the Kiev military said Tuesday.

Ukrainian military spokesman Oleksandr Motuzyanyk said fighting along the 500-kilometer (300)-mile) front had "escalated", accusing the insurgents of violating a shaky truce agreement on 79 occasions since Monday afternoon.

A second top Ukrainian official in the space of two weeks quit Monday in protest at alleged graft that critics say pro-Western President Petro Poroshenko has done little to fight.

Deputy Prosecutor General Vitaliy Kasko tendered his resignation and accused his boss Viktor Shokin of turning the law enforcement body into one where "corruption reigns".

Sanctions against Russia will continue until it implements all aspects of the Ukraine peace agreement reached in Minsk last year, U.S. Secretary of State John Kerry said Saturday.

"Russia has a simple choice: fully implement Minsk or continue to face economically damaging sanctions," he said at the Munich Security Conference in Germany.

A Ukrainian soldier was killed in a sudden upsurge in clashes between pro-Russian rebels and government forces in the country's separatist east, Ukraine's military said Friday.

"Over the past 24 hours, as a result of hostilities one Ukrainian soldier was killed and another three were wounded," military spokesman Andriy Lysenko told reporters.

Ukraine's president Monday accused Russia of sending troops and weapons into the ex-Soviet state's conflict-torn east, and warned that a fragile peace deal was not being fully implemented.

"It's terrible that after the Minsk agreement ... we still face serious security problems in the Donbass," Petro Poroshenko said in reference to the pro-Russian separatist region, ahead of talks in Berlin with German Chancellor Angela Merkel.

Clashes between pro-Russian rebels and government forces in eastern Ukraine have dwindled but villages in the buffer zone around the frontline have been surreptitiously changing hands, raising fears that the tenuous ceasefire could unravel.

Ukraine's blue and yellow flag was hoisted in December in the sleepy village of Pavlopil, which for months had remained trapped in the "grey zone" between Kiev-controlled and rebel territory.

A Russian court on Tuesday sentenced a Ukrainian man to 11 years in prison on espionage charges, after he was accused of buying "state secrets" concerning Russia's aerospace industry.

The verdict against Valentyn Vygovskiy, described in the media as a 32-year-old businessman, was swiftly condemned by Kiev, with foreign ministry spokeswoman Maryana Betsa calling it "illegal".

It used to take Vadym two hours to drive to see his relatives in east Ukraine but now the journey involves days waiting in the freezing cold at government checkpoints to leave rebel-held territory.

The miner, 37, had camped out for two days with his wife and seven-year-old son in their ageing Lada car -- joining hundreds of vehicles queued up on one of the country roads leading out of the pro-Russian fiefdom separatists have carved out.

U.S. Vice President Joe Biden said during a visit to Kiev on Monday that Washington remained committed to its demands for Russia to adhere to a shaky Ukrainian peace agreement and hand back Crimea to Kiev.

The broad February deal struck in the Belarussian capital Minsk "cannot succeed if Russia does not fulfill its commitments," Biden said after talks in the Ukrainian capital with President Petro Poroshenko.

Tensions between Moscow and Kiev ratcheted up further Wednesday as Ukraine decided to stop buying gas from Russia and closed its airspace to its giant eastern neighbor’s airlines.

The two measures not only underscore the acrimonious nature of the former Soviet countries' relations but also highlight how difficult one of Europe's deadliest crises since the Balkans Wars of the 1990s may be to resolve.