The United Nations is determined to hold Libya's national conference on possible elections on time despite eastern forces' advance on the capital Tripoli in an escalation of the conflict, a senior UN envoy said today.

G7 foreign ministers warned eastern Libyan military commander Khalifa Haftar to halt his thrust on Tripoli, menacing the internationally recognised government based there, or face possible international action.

Mr Haftar's Libyan National Army (LNA) said yesterday its forces had advanced into the capital's southern outskirts and taken its former international airport.

The offensive by the LNA, which is allied to a parallel administration based in the main eastern city of Benghazi, intensifies a power struggle that has fractured the oil-producing country since the 2011 overthrow of Muammar Gaddafi.

The United Nations aims to stage a conference in the south-western town of Ghadames on 14-16 April to weigh elections as a way out of the country's prolonged factional anarchy, which has seen Islamist militants establish a toehold in some areas.

Ghassan Salame, the UN special envoy to Libya, said he was striving to prevent the new crisis from getting out of control.

"We have worked for one year for this national conference, we won’t give up this political work quickly," he said.

"We know that holding the conference in this difficult time of escalation and fighting is a difficult matter. But we are determined to hold it on time unless compelling circumstances force us not to," he told reporters in Tripoli.

At a G7 meeting in France, German Foreign Minister Heiko Maas said he and his counterparts had agreed they must exert pressure on those responsible for the upsurge of fighting in Libya, especially Mr Haftar.

Italian Foreign Minister Enzo Moavero Milanesi said the G7 statement "should be seen as an appeal to General Haftar to consider with a sense of responsibility peaceful solutions for stabilising Libya under the UN umbrella".

Russia also called for restraint, saying Deputy Foreign Minister Mikhail Bogdanov told Mr Haftar in a phone call that Moscow continues to insist on a political solution to "disputed issues" in Libya.

Mr Haftar told Mr Bogdanov about what he described as efforts to fight terrorists in Libya, including near Tripoli, according to the Russian Foreign Ministry.