Pro-Ankara Syrian militia fighters deployed in Libya have started to withdraw, a French diplomatic source said on Thursday, describing the move as "encouraging".

"This is one of the encouraging gestures that we have seen. There are movements (of the Syrian militia forces) that have been seen and noted," said the source, who asked not to be named.

The Syrian militia forces' presence in Libya alongside Turkish troops has repeatedly angered France, which has called for all mercenaries to leave the country including Russians.

"Our message is that this must continue, not only with the Syrian mercenaries but with all foreign forces," added the source.

The source said there had been discussions on the issue between Libya's new interim prime minister Abdul Hamid Dbeibah and the Turks.

The source added that statements from the Turkish side indicated Syrian fighters may have been flown out of the country but this needed to be "specified and confirmed".

Libya's new unity government earlier Thursday demanded the "immediate" withdrawal of all foreign mercenaries, in a call backed by the visiting top diplomats of France, Germany and Italy.

Oil-rich Libya has been torn by civil war since a NATO-backed uprising led to the toppling and killing of dictator Moamer Kadhafi in 2011.

A new transitional executive emerged from a UN peace process that was launched in November in Tunis, then voted on in Geneva and confirmed by Libya's parliament on March 10.

France has been bitterly critical of Turkey's military intervention in Libya, while rejecting claims that Paris had secretly favoured Libya's eastern strongman Khalifa Haftar in the conflict.

Receiving the visiting chairman of Libya's Presidential Council Mohammad Menfi in Paris on Tuesday, President Emmanuel Macron insisted that the withdrawal of all foreign forces was a priority.

"We must do everything so that foreign forces who are on the ground in Libya leave as soon as possible," said Macron.

"Turkish and Russian fighters, those foreign fighters sent in by them or others, must leave Libya as soon as possible as only the Libyan armed forces are legitimate," he said.