Clashes with Russian-backed rebels killed six Ukrainian soldiers on Thursday, officials said, in the bloodiest surge in fighting along the volatile front line in recent months.

The violence flared several days after a top rebel leader announced a plan to form a new "state" that Kiev warned could put a long-stalled peace plan further in jeopardy.

Ukrainian military spokesman Andriy Lysenko said rebel shelling killed six servicemen and wounded two more around the insurgents' de-facto capital Donetsk.

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Three more soldiers were killed and three injured when their vehicle drove over a mine north-west of the second biggest rebel city, Lugansk, Lysenko announced.

On the other side of the front, a separatist news agency accused Kiev of wounding three civilians in a bombardment by heavy weaponry.

On Wednesday the army, in a statement, announced the death of two soldiers in bombardments 40 kilometres from Donetsk.

Ukraine and the West say Russia has funnelled troops and arms across the border to fan the flames of war in Europe's backyard.

Moscow has denied the allegations despite overwhelming evidence that it has been involved in the fighting and its explicit political support for the rebels.

US Department of State spokeswoman Heather Nauert said Washington condemned the latest violence in eastern Ukraine.

"The last 24 hours were considered the deadliest one-day period in 2017," she said, calling for "Russia and the forces that it arms, trains and leads in the east, to immediately observe the ceasefire".

Nauert says the United States calls on those forces to allow international monitors to have "full, safe and unfettered" access to the conflict zone.

Rebels propose new state of 'Little Russia'

Donetsk rebel chief Alexander Zakharchenko on Tuesday announced plans to create a new "state" to replace Ukraine.

The proposed country was to be called Malorossiya (Little Russia) - a Tsarist-era term for an area covering much of modern day Ukraine - and have its capital in Donetsk.

The plan appears to be dead in the water after other rebel bosses rejected it and the insurgents' backers in the Kremlin dubbed it a "private initiative".

However, it sparked fears it could drive another nail into the coffin of a stalled peace process that has failed to end a conflict that has claimed 10,000 lives.

A deal brokered by the leaders of Ukraine, Russia, France and Germany in the Belarusian capital Minsk in 2015 has hit a wall but is still viewed by those involved as the only way of unwinding Ukraine's war.