United Nations (UN) Secretary-General Antonio Guterres on Monday appointed US diplomat Stephanie Williams to lead mediation efforts in Libya after his special envoy quit just weeks ahead of planned elections in the war-torn country.

UN special envoy on Libya, Jan Kubis, is due to step down on Friday. Guterres had informally suggested veteran British diplomat Nicholas Kay as a replacement, but Russia said it would not support Kay, according to diplomats. The 15-member UN Security Council, operating by consensus, must approve a new appointment.

Guterres named Williams as his special adviser, which does not require council approval. Williams was the acting special envoy on Libya after Ghassan Salame quit in March 2020 because of stress and before Kubis was approved in January 2021.

Kubis, who has been based in Geneva, said last month there was a need for the head envoy to be based in Libya's capital Tripoli and he resigned to "to create conditions for this".

Williams "will lead good offices and mediation efforts and engagements with Libyan regional and international stakeholders to pursue implementation of the three intra-Libyan dialogue tracks - political, security and economic - and support the holding of presidential and parliamentary elections in Libya," UN spokesperson Stephane Dujarric said in a statement.

Libya descended into chaos after the NATO-backed overthrow of longtime autocrat Muammar Gaddafi in 2011. In October last year, the two major sides in Libya's civil war - the internationally recognized Government of National Accord and Khalifa Haftar's eastern-based Libyan National Army - agreed a ceasefire.

A UN political forum last year demanded parliamentary and presidential elections take place on December 24 as part of a roadmap to end the war. However, disputes over the planned vote threaten to derail the peace process.

A first-round presidential vote is set for December 24 and the parliamentary election has been delayed to January or February. However, rules for the elections have not yet been agreed.