Libya's interim government says its forces have captured the airport and other locations in the southern desert city of Sabha, one of ousted leader Moammar Gadhafi's last remaining strongholds.

A military spokesman for the National Transitional Council, Colonel Ahmed Bani, said Monday that NTC flags are flying over the airport, an old Italian fort and other strategic buildings inside Sabha.

The city, 650 kilometers south of the capital, Tripoli, controls the main trail to neighboring Niger – an escape route used by members of Mr. Gadhafi's entourage.

Any advance on Sabha would be an important boost for Libya's revolutionary forces. NTC fighters are struggling to oust pro-Gadhafi loyalists from the towns of Bani Walid and Sirte, and to contain disunity within their ranks.

In New York, the United Nations said it has chosen British diplomat and rights activist Ian Martin to head its new mission in Libya.

Martin is a former secretary general of Amnesty International and a former U.N. special envoy in Nepal. He is expected to lead up to 200 U.N. staff with an initial three-month mandate to help with a range of tasks from electoral assistance to police training.

Earlier Monday, pro-Gadhafi forces repelled NTC fighters with heavy gunfire near the northern entrance to heavily fortified Bani Walid.

Volunteer fighters fled in chaos from the city Sunday when loyalist troops attacked their positions with mortars and sniper fire. Regular, trained provisional authority troops had pulled away from Bani Walid after failing to take the town.

In Mr. Gadhafi's hometown of Sirte Sunday, revolutionary forces also encountered fierce resistance.