Ukraine said it had repelled a tank assault by pro-Russian rebels that threatened to halt a shaky ceasefire and escalate the 16-month conflict in the breakaway east of the country.

The Ukrainian military said that up to 400 rebel fighters supported by tanks had attacked government forces around the village of Starohnativka, 50 km (30 miles) north of the Kyiv-held port city of Mariupol.

Control of Mariupol could help the rebels form a corridor to the Crimea peninsula, which Russia annexed from Ukraine last year. Military spokesman Yaroslav Chepurny said the incident showed that Mariupol city remained under threat from separatists.

"The main danger is in the further approach routes to the city - from the direction of Granitnoe, Starohnativka. We knew about this and were prepared," he said.

Ukrainian authorities warn of imminent escalation

President Petro Poroshenko meanwhile announced that about "200 insurgents used tanks to storm" Novolaspa - a nearby village halfway between the separatists' stronghold Donetsk and Mariupol. It was unclear whether some of the rebels from the Starohnativka assault were also part of the Novolaspa attack.

"Ignoring the truce agreements, our enemies are continuing to stage provocations that are meant to escalate the conflict," the Ukrainian defense ministry said in a statement. The Ukrainian foreign ministry called the clashes "a dangerous indication of a further escalation to come".

Pro-Russian rebels deny attack

The rebels denied attacking government troops. They also denied that the push took place and signaled that they had always had militia units stationed in areas surrounding Novolaspa.

"The armed forces of Ukraine simply put the village under a heavy shelling attack," a local separatist official told the rebels' main news site.

"Novolaspa remains under the control of the People's Republic of Donetsk."

Frail and shaky ceasefire

The two self-proclaimed republics of Donetsk and Lugansk launched their revolt shortly after the February 2014 ouster of a Moscow-backed president in Kiev and Russia's subsequent seizure of Ukraine's Crimea peninsula.

A ceasefire deal, signed in mid-February of 2015, has failed to stem the violence in the eastern conflict zone. Both sides regularly accuse the other of violating the terms of the peace agreement. Casualties are reported almost daily.

Germany Chancellor Angela Merkel and French President Francois Hollande have held teleconferences with Russian President Vladimir Putin and Ukrainian President Petro Poroshenko in the past month aimed at reviving the peace process.

According to UN estimates, more than 6,500 soldiers, separatists and civilians have been killed since the fighting erupted.

ss/rc (Reuters, AFP)