By Joseph Logan and Stephanie Nebehay
TRIPOLI/GENEVA (Reuters) - The United Nations is negotiating with Libya's government, rebels and NATO to stop fighting for up to three days to allow food and medical supplies to reach civilians, its envoy said Wednesday.
In a separate development, a Tunisian security source said Libyan leader Muammar Gaddafi's daughter Aisha and her mother Safia had crossed into Tunisia four days ago and were still on its southern island of Djerba.
The purpose of their trip was not clear.
Besides the proposed cease-fire, Panos Moumtzis, U.N. humanitarian coordinator for Libya, said he would also seek security guarantees for U.N. aid workers to reach the rebel-held city of Misrata and the Western Mountains in talks with authorities in the Libyan capital Friday.
Misrata is a key battleground in the three-month-old war in which rebels fighting to topple Gaddafi have seized control of the oil-producing east of the country, aided by a NATO bombing campaign to enforce a U.N. mandate to protect civilians.
The war is deadlocked on its main front but fighting flared again in the besieged port city in western Libya and seven people were killed, most of them rebel fighters, in clashes on Tuesday with government forces, a hospital doctor said.
Moumtzis said the pause could last one to three days. While not a formal cease-fire, it would help evacuate migrants, wounded and others wishing to leave areas riddled by fighting.
"The humanitarian pause is driven by humanitarian principles and the need to be able to provide urgently needed life-saving assistance to the civilian population," he told a news conference, declining to give a target start date for the pause.
A NATO official said the trans-atlantic alliance would cooperate with the United Nations over humanitarian aid.
"We all share the U.N. interest in making sure humanitarian supplies reach Libya," said the official. "NATO is enabling humanitarian assistance going to Libya. We are not obstructing humanitarian assistance, just the opposite."
MYSTERY TRIP
The United Nations withdrew its international staff from Tripoli on May 1 after its offices were ransacked on the day Libya said Gaddafi's youngest son and three grandchildren were killed in a NATO air strike.
Mystery surrounded the trip to Tunisia by Aisha, a staunch public supporter of Gaddafi's policies, and her mother. The Tunisian source told Reuters they came with a Libyan delegation.
Earlier, Libyan rebels and a Tunisian security source said the head of Libya's National Oil Corporation, Shokri Ghanem, had defected and fled to Tunisia. If confirmed, the defection would undermine Gaddafi's efforts to cling to power.
The war's diplomatic fallout intensified. Tunisia threatened to report Tripoli to the U.N. Security Council if it fired into Tunisian territory again, after government shells landed across the frontier near the Dehiba-Wazzin crossing Tuesday.
At least four Russian Grad rockets fired from Libya landed inside Tunisia, according to a Reuters reporter at the scene on Tuesday. Wednesday, the shelling appeared to have stopped and the border had reopened.
Farmers were crossing over from Libya to take livestock to a market on the Tunisian side, while a Tunisian military helicopter was making passes around the area.
The border crossing is a lifeline for rebels in the Western Mountains region front, allowing food, medicine and fuel to reach rebel-held towns on the mountain plateau, and ambulances to take casualties to hospital in Tunisia.
In the east of Libya, Al Jazeera television reported that NATO aircraft had bombed Gaddafi forces in an area west of the rebel-held city of Ajdabiyah.
It also said rebels had seized two crossing points, one between Libya and Chad, and the other between Libya and Sudan.
The International Criminal Court prosecutor, Luis Moreno-Ocampo, said Libya must abide by its agreements as a U.N. member and not dismiss international efforts to bring Gaddafi and others to justice.
Moreno-Ocampo requested arrest warrants Monday for Gaddafi, his son Saif al-Islam, and spy chief Abdullah al-Senussi, who is Gaddafi's brother-in-law, on charges of crimes against humanity -- a request Tripoli denounced.
"I wish to remind you that the situation in Libya was referred to the Office of the Prosecutor by the United Nations Security Council," Moreno-Ocampo said in a letter to Libyan foreign minister Abdelati Obeidi which he released to the media.
Earlier Wednesday, Libya freed four foreign reporters who had been charged with entering the country illegally and said it was hard for its army to distinguish between journalists and people working with rebels.
(Reporting by Joseph Nasr, Matt Robinson, Sylvia Westall, David Brunnstrom, Tarek Amara and Allan Dowd; writing by Matthew Bigg and Maria Golovnina; editing by Mark Trevelyan)
