The so-called Libya Quartet hopes to end the years-long conflict in the African nation.
Libya’s internationally recognised government in Tripoli has declared a military alert after eastern forces loyal to renegade General Khalifa Haftar said they had moved towards the western part of the country.
A brief skirmish between Haftar’s self-styled Libyan National Army (LNA) and forces allied to Prime Minister Fayez al-Serraj, the head of the Tripoli-based administration, was reported on Wednesday evening near a town south of the capital.
“Right now they [there] are clashes south of Tripoli … in Gharyan,” LNA spokesman Ahmed Mismari told the UAE-based al-Arabiya channel.
No casualty figures or details were immediately available.
The oil-rich country, which has been in turmoil since the NATO-backed removal of its long-time ruler Muammar Gaddafi in 2011, has at least two rival administrations: one based in Tripoli, and another in the eastern city of Tobruk, which is allied with Haftar.
In recent years, Haftar has expanded his foothold in large parts of Libya and has repeatedly expressed his intention to march on Tripoli.
Earlier on Wednesday, the LNA’s media centre said on Facebook that several units had headed “to the western region to cleanse it of the remaining terrorist groups”.
Its statement gave no details, but the area appears to be the coastal road linking the eastern city of Benghazi, the LNA’s main base, with Tripoli in western Libya. An accompanying video showed a column of dozens of armed vehicles moving along a road, but it was not immediately possible to identify their location or destination.
Serraj, who relies on patches of armed groups with flexible loyalties, called the eastern advance an “escalation” and urged Haftar’s forces to “stop using the language of threats”.
He said he had ordered pro-government forces to prepare to “face all threats … whether from terrorist groups, criminals, outlaws and all who threaten the security of every Libyan city”.
‘De facto commander’
Reporting from Tripoli, Al Jazeera’s Mahmoud Abdelwahed said the LNA’s positioning is a display of strength.
“Many analysts and military experts in Libya say that this is some kind of show of force or power. Haftar’s forces are using that to, first of all, intimidate their adversaries in the west of the country, and secondly, to test their power,” he said.
“They want to see who wants to confront them in the western area.”
The United Nations is holding a conference this month in the southwestern city of Ghadames to discuss a political solution to prepare the country for long-delayed elections and avoid a military showdown.
Abdelwahed said it was possible that Haftar wants to reach Tripoli before the conference “so he could impose himself as a de facto security commander in the western area”.
Analysts doubt the LNA is capable of launching a full-scale attack as it has stretched itself with the southern advance and is reliant on tribesmen and other auxiliary forces.