Ukranian and rebel envoys concluded "difficult" marathon talks on Wednesday without agreeing the date of a new round aimed at ending the pro-Russian uprising devastating the ex-Soviet state's industrial east.

The five-hour preliminary discussion in the Belarussian capital Minsk had been tentatively due to be followed by a second meeting on Friday at which a final accord was to be signed.

But rebel representatives stressed that they could not yet promise whether the negotiations would resume as planned.

"We had a difficult preliminary meeting," Donetsk separatist region mediator Denis Pushilin told a pro-rebel news site.

"The date and time of the next meeting is still up in the air. It is under discussion," Pushilin said.

The self-declared Donetsk leader Alexander Zakharchenko later added vaguely that "a second round is still ahead."

Neither Ukrainian officials nor the Russian and European envoys spoke to reporters waiting outside the gated Belarussian state mansion that hosted the talks.

Sharply contrasting visions of Ukraine's place in Europe and its system of government have been persistently blocking a solution to the eight-month-old war.

The two Russian-border provinces of Lugansk and Donetsk rose up against the historic shift toward Europe that Kiev made in the wake of last winter's ouster of an unpopular Moscow-backed president.

The separatist commanders have since declared their own republics and will settle for no less than Ukraine becoming a loose federation in which they manage most of their own affairs.

This option is backed firmly by Russia but rejected by Ukrainian nationalists who make up an important part of President Petro Poroshenko's government.

Ukraine has remained tightly centralised since independence and is only now considering easing its hold over the country's regions in order to stem public resentment over the relative prosperity enjoyed in Kiev.

Such problems helped undermine two deals reached in Minsk in September that Poroshenko was forced into following a surprise rebel counteroffensive that NATO believes was backed by crack Russian forces and tanks.

Russian President Vladimir Putin flatly calls soldiers who crossed into the Ukrainian war zone volunteers who are "answering the call of the heart".

A UN count puts the number of deaths following an initial September 5 truce deal at more than 1,300.

The overall toll since last winter from Europe's bloodiest conflict since the 1990s is more than 4,700 and has caused friction between many of the country's Ukrainian and Russian speakers that may take generations to heal