G7 foreign ministers warned eastern Libyan military commander Khalifa Haftar to desist from his thrust on the capital Tripoli, menacing the internationally recognised government there, or face possible international action.
Ghassan Salame was speaking to reporters in Tripoli a day after Haftar's forces said they had advanced into the capital's southern outskirts and taken its former international airport.
The offensive by Haftar's Libyan National Army (LNA), allied to a parallel administration based in the eastern city Benghazi, escalated 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 southwestern town of Ghadames on April 14-16 to weigh elections as a way out of the country's factional anarchy, which has seen Islamist militants establish a toehold in some areas.
Salame, then 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."
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 intensification of fighting in Libya, especially Haftar.
Italian Foreign Minister Enzo Moavero Milanesi said Haftar must heed international warnings to halt his advance on Tripoli or else "we will see what can be done".