Libyan government fighters have captured Muammar Gaddafi’s son Mo’tassim as he tried to escape the battle-torn city of Sirte, National Transitional Council (NTC) officials said.

The capture of the deposed leader’s national security adviser, and the first member of the Gaddafi family, is a big boost to Libya’s new rulers, whose forces are still battling pro-Gaddafi fighters in his home town of Sirte.

“He was arrested yesterday in Sirte,” Col. Abdullah Naker told Reuters on Wednesday, while other NTC sources said Mo’tassim was taken to Benghazi where he was questioned at the Boatneh military camp where he is being held.

He was not injured but exhausted. Hundreds of NTC fighters took to the streets in several Libyan cities and fired shots in the air in celebration.

Gaddafi loyalists have fought tenaciously for weeks in Sirte, one of just two major towns where they still have footholds, two months after rebels seized the capital Tripoli.

“We have control of the whole of the city except neighbourhood ‘Number Two’ where the Gaddafi forces are surrounded,” said Khaled Alteir, a field commander in Sirte.

Green flags, the symbol of Gaddafi’s 42 years in power, still fly above many of the buildings there.

NTC forces were using tank and artillery fire, as well as snipers, to soften up the remaining pocket of resistance before launching a ground assault.

There was no urgency to launch the final attack, Alteir said, as there could still be civilians in the area.

NTC infantry cleaned their weapons and moved up to the front line in Sirte yesterday, but were met by sniper fire and rocket-propelled grenades from the Gaddafi side, indicating resistance had not crumbled after the capture of Mo’tassim.

Mo’tassim is seen as belonging to a conservative camp, rooted in the military and security forces which resisted his brother Saif al-Islam’s reform attempts.

A senior NTC military official told Reuters that Mo’tassim had cut his usually long hair shorter to disguise himself.

Gaddafi and his most politically prominent son, Saif al-Islam, have been on the run since the fall of Tripoli in August, and the former Libyan leader is believed to be hiding somewhere far to the south in the vast Libyan desert.

His daughter Aisha, her brothers Hannibal and Mohammed, their mother Safi and several other family members fled to Algeria in August and have lived there since, while another son, Saadi, is in Niger.

In the “Number Two” neighbourhood, government forces found 25 corpses wrapped in plastic sheets, and have accused pro-Gaddafi militias of carrying out execution-style killings.