(Reuters) - Germany, Russia, Ukraine and France have decided not to hold a summit on the conflict in Ukraine on Thursday because of a lack of progress in implementing a four-month-old ceasefire agreement.

The Russian, German and French leaders had been invited to talks in the Kazakh capital Astana by Ukrainian President Petro Poroshenko.

But the four countries' foreign ministers said in a joint statement after talks in Berlin late on Monday that the failure to implement the truce effectively, 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 that only a strict ceasefire could pave the way for the leaders to meet in Astana, adding: "It was agreed that it was necessary to work more on this."

Lavrov said there would be another meeting of the so-called Contact Group - Russia,Ukraine, Europe's OSCE security watchdog and pro-Russian rebels - to find ways to implement the ceasefire that was agreed in the Belarussian capital Minsk in September.

German Chancellor Angela Merkel said last week that Europe could not consider lifting sanctions against Russia until all elements of the 12-point Minsk agreement were implemented.

The conflict has pushed relations between Russia and the West to their lowest ebb since the Cold War ended.

Kiev and Moscow blame each other for the failure to implement the Minsk deal and end the conflict in east Ukraine, where more than 4,700 people have been killed since mid-April in fighting between the rebels and the Ukrainian army.

Lavrov also said there was a greater understanding that all parties in Ukraine should be involved in talks about constitutional change. Moscow has pressed hard for the self-declared separatist administrations in the eastern regions of Donetsk and Luhansk to have a say over the future of Ukraine.

Kiev does not recognize the local leaders in the two regions.