Russia and Turkey postponed talks on Libya due to discord over the Ankara-backed Tripoli government's push to retake the key coastal city of Sirte from eastern forces, a Turkish official said on Monday.

With Turkish military support, Libya's internationally recognised Government of National Accord (GNA) has advanced for weeks against Khalifa Haftar's Libyan National Army (LNA), which is backed by Russia, the United Arab Emirates and Egypt.

Heavy clashes have emerged as the GNA has laid siege to LNA-held Sirte, close to major energy export terminals on the Mediterranean seaboard.

Russian Foreign Minister Sergei Lavrov did not make the trip to Istanbul on Sunday for a scheduled meeting with his counterpart, Mevlut Cavusoglu, but the Turkish official said background talks were continuing at expert level.

"A result was supposed to come out (of the meetings), but that stage could not be reached. There are issues where the two countries are on opposing sides," the Turkish official said on condition of anonymity.

"One of the main issues for the postponement of the Lavrov visit is the (GNA's) plan for an operation into Sirte...which has emerged as a target."

The Kremlin did not comment. Russia's foreign ministry said on Sunday it was pursuing a "prompt ceasefire" and that Lavrov would reschedule the meeting. Cavusoglu said on Monday the postponement was unrelated to any issues on "core principles".

The United Nations said last week the warring sides had begun new ceasefire talks in Libya after GNA forces repelled a protracted LNA assault on the capital Tripoli.

Sirte, about halfway between GNA-held Tripoli and LNA-held Benghazi, is the closest city to Libya's main energy export terminals. Haftar's forces seized the city in January and the conflict's new front line has emerged just to the west.

*This story was edited by Ahram Online.