Ukraine, Russia, Germany and France have scrapped plans for a summit this week on the conflict in Ukraine because of a lack of progress in implementing a four-month-old cease-fire agreement.
With bloodshed continuing between Ukrainian government forces and Russian-backed separatists, there was no sign when a new attempt at a big-power meeting could be scheduled.
The Russian, German and French leaders had been invited to talks on Thursday in the Kazakh capital Astana by Ukrainian President Petro Poroshenko on ending the conflict in Ukraine's east that has killed more than 4,700 people in nine months.
But the four countries' foreign ministers said after talks in Berlin on Monday the failure to implement the truce fully, and the need to agree on how to deliver aid and free prisoners, meant "further work needs to be done" before a summit is held.
"The differences in opinion made it clear how difficult it is to make progress towards a political solution or a summit in Astana, from which much is expected and which must be prepared," German Foreign Minister Frank-Walter Steinmeier told reporters.
Russian Foreign Minister Sergei Lavrov said all had agreed only a strict cease-fire could pave the way for the leaders to meet in Astana.
The separatists and government forces blame each other for regular violations of the cease-fire which technically came into force in early September as part of a 12-point peace protocol signed in Minsk, capital of Belarus.
Attacks on Donetsk Airport
Kiev says more than 200 Ukrainian service personnel have been killed in attacks by separatists since the truce was declared, including nine since the start of this year.
The Ukrainian military said separatists used Grad multiple rocket launchers in 26 separate attacks on Ukrainian positions overnight, including on the international airport in the city of Donetsk over which government forces have tenuous control.
Military spokesman Andriy Lysenko said the rebels had used anti-aircraft systems and artillery against the airport's flight control tower, destroying it down to the fifth floor.
A separatist leader, Denis Pushilin, blamed the postponement of the summit on Kiev's "non-constructive position," including not recognizing the "special status" of the separatist-held regions in the east, Interfax news agency reported.
Lavrov said there would be another meeting of the Contact Group — Russia, Ukraine, Europe's OSCE security group and pro-Russian rebels — to find ways to implement the Minsk agreement.
Meetings of this group have often proved difficult to arrange, with Kiev unwilling to confer recognition on separatist leaders by meeting them officially.
German Chancellor Angela Merkel said last week Europe could not consider lifting sanctions against Russia until all elements of the Minsk agreement were implemented. Ukraine says Russia has not carried out part of the agreement calling for the withdrawal of troops, armed groups and military equipment.