Taking Sirte would bring Libya's new rulers a big step closer to establishing control of the entire, sprawling North African country almost two months after they seized the capital Tripoli, but Qaddafi loyalists are putting up stiff resistance.

National Transitional Council (NTC) forces captured Sirte's main hospital as well as the university and a lavishly built international conference center, but one large group of anti-Qaddafi fighters fled in disarray after coming under an intense mortar volley.

The university, seized overnight, also came under a sustained counterattack, as did the conference center.

While Reuters correspondents had entered the landmark buildings, confirming their capture, intense fighting was now under way around all three buildings and it was unclear whether they were still in provisional government hands.

One large group of NTC fighters, approaching Sirte from the west and trying to pin down Qaddafi loyalists against the sea, saw their advance turn into a rout under heavy and accurate mortar bombardment.

Their trademark pick-up trucks raced to the rear, some running on flat tires. One man with arm and leg wounds frantically limped away from the carnage as commanders screamed over the radios for their troops to pull back.

Soon seven fighters lay dead in a field hospital while dozens more were being treated for shrapnel wounds.

The speed of the reversal underlines the fragility of government advances and the weight of the task still ahead of them despite their leaders' upbeat assessment of the fight.

NTC Chairman Abdel Jalil told a news conference in the capital Tripoli that his men had reached Sirte city center and were combing the town for snipers.

The only other major Qaddafi-held town, Bani Walid, in the desert far to the south, was also under government siege from no fewer than five sides, he said.