Libya's warring sides have begun to engage in a new round of ceasefire talks, the United Nations said on Wednesday, after rapid gains by the UN recognised government ended with heavy fighting around the central coastal city of Sirte.

The Government of National Accord (GNA), which is backed by Turkey, and the eastern-based Libyan National Army (LNA), backed by the United Arab Emirates, Russia and Egypt, have each met separately with UN negotiators, the UN Libya mission said.

It follows the sudden collapse of the LNA's 14-month offensive to capture the capital Tripoli, the seat of the GNA, and its retreat from most of its territory in northwest Libya.

A GNA effort from Monday to push further east and capture Sirte, effectively wiping out all the LNA's gains since the start of its Tripoli campaign in April 2019, was repulsed with air strikes, an LNA military source said.

"The Mission is particularly concerned by reports of escalation and mobilization in and around the city of Sirte", the UN Libya mission said in its statement announcing the talks were under way.

It said it had verified at least 19 civilian deaths in Sirte.