Negotiations mediated by European and Russian envoys in the Belarussian capital Minsk broke up after more than five hours yesterday with the pro-Russian rebels reporting little to no progress.
That scuppered a plan for both sides to gather again on Friday in order to sign a comprehensive agreement reinforcing a September 5 truce deal that was followed by 1,300 more deaths.
The chief negotiator for eastern Ukraine's self-declared Donetsk People's Republic said he could not disclose the "difficult" talks' details because all sides preferred to keep their differences private.
"We had a difficult preliminary meeting," Denis Pushilin told a pro-rebel news website.
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"The date and time of the next meeting is still up in the air. It is under discussion," Pushilin said. The Belarussian foreign ministry added that "the date of the next meeting will be announced later".
Neither Ukrainian officials nor the Russian and European envoys spoke to reporters waiting outside the gated Belarussian state mansion that hosted the negotiations.
Sharply contrasting visions of Ukraine's place in Europe and its system of government have been persistently blocking a solution to the protracted war.
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.