Colonel Hamed Al-Hasi, commander of the anti-Gaddafi Qatar brigade east of Sirte, said the truce had expired on Sunday, but a full assault was still out of the question.

"There are still more than 15,000 persons inside. We cannot sacrifice them," he said.

He said NTC forces would for now instead be targeting the district of Bouhadi, just to the south of Sirte. He said a captured Gaddafi loyalist had told them that Mo'atassem, one of the deposed Libyan leader's sons and his former national security adviser, was in the district.

"We've been concentrating since last night on Bouhadi," he said. "If we catch Mo'atassem, things will calm down."

NATO warplanes were more in evidence than in previous days, with aircraft flying unusually low over the city. In one 10-minute period there was a steady rumble from bombs dropping on an area south of the city, a Reuters reporter said.

Libyans ended Gaddafi's 42-year rule in August when rebel fighters stormed the capital. Gaddafi and several of his sons are still at large, and his supporters hold Sirte and the town of Bani Walid, south of Tripoli.

Gaddafi's supporters are too weak to regain power, but their resistance is frustrating the new rulers' efforts to start building the post-Gaddafi Libya.

While it needs to stamp out the last resistance swiftly, the NTC does not want to use indiscriminate shelling, which could hurt its standing and make it even harder to govern Libya's fractious tribes and regions.