Muammar Gaddafi diehards fought pitched battles on Thursday with combatants loyal to Libya's new rulers for control of the ousted leader's birthplace Sirte, with the heaviest fighting focused at the port.

Several anti-Gadhafi fighters who were injured amid heavy shelling in Sirte wait as they are transported in Red Crescent helicopters from Ras Lanuf to Benghazi on Wednesday.?[China Daily via Agencies]

World police body Interpol, meanwhile, issued an arrest notice for Gaddafi's son Saadi for alleged crimes while head of Libya's football federation.

Fighters of the National Transitional Council (NTC), Libya's interim government, had been forced to retreat during ferocious fighting on the eastern edge of Sirte that raged through the night, their commanders said.

But after a brief lull, they returned to the fray in the early afternoon, an AFP reporter said, with the two sides shelling each other and trading heavy machine-gun fire around the port as well as near the Mahari Hotel.

NTC field commanders said their forces remained in control of the hotel and the port, which they overran on Tuesday, but that the situation was fluid.

"It is becoming a day-to-day fight. One day we are winning, the next day they are winning," said one commander who asked not to be identified, even as NATO warplanes flew overhead.

The commander told AFP the latest fighting in Sirte was the fiercest yet since NTC forces launched their assault on the Mediterranean city, 360 kilometers east of Tripoli, on Sept 15.

"They (Gaddafi loyalists) have lost everything. This is their last battle and so is fighting fiercely. Our troops are taking a heavy beating at the moment. Today we retreated 3 km," he said before the fighting resumed in earnest.

In a separate incident, three NTC fighters were killed by "friendly fire" on Wednesday when they were shelled by a tank on the frontline in eastern Sirte, the commander said.

"There was some lack of coordination and ... our fighters were hit by a shell fired by our tank stationed behind them. There were three martyrs," he said.

Further casualties were suffered, according to medics at a field hospital about 50 km west of Sirte, when a rocket in a munitions dump fired accidentally and hit a room filled with fighters, killing two and wounding 18.

Equally fierce resistance from loyalists in the desert town of Bani Walid, Gaddafi's other remaining bastion of support, has stalled a final assault by NTC fighters, said commanders, who urged NATO to increase its air support.

While the fugitive Gaddafi's whereabouts remain unknown, Libya's defense ministry spokesman Ahmed Bani said in Tripoli that one of his sons, Seif al-Islam, was in Bani Walid and another, Mutassim, in Sirte.

Along with his father and former intelligence chief Abdullah al-Senussi, Seif is wanted by the International Criminal Court for alleged crimes against humanity.

The anti-Gaddafi forces have urged NATO to intensify its air war because of the heavy casualties they are suffering.

An AFP correspondent said that despite using tanks, rocket launchers and artillery, the NTC forces had not advanced from positions held for the past few days in Bani Walid, 170 km southeast of Tripoli.

"There is always incoming missile and artillery fire. We are returning fire with heavy weapons but we are not sending in infantry. We are waiting for reinforcements," Captain Walid Khaimej told AFP.

"NATO is here but is not doing enough. They take out the rocket launchers firing at us, but they are immediately replaced. We need more help from NATO."

Under a UN mandate aimed at protecting civilians, the alliance has been giving air support to the revolt that erupted in February and forced Gaddafi out of Tripoli and into hiding last month.

Colonel Roland Lavoie, the air campaign's military spokesman, denied NATO had reduced its activity in Libya, noting alliance aircraft had conducted at least 100 sorties a day over the past few days.

"The number of strikes depends on the danger against the civilian population, in conformity with our mandate," Lavoie told AFP.