Although an opposition spokesman told The Washington Post that 10 fighters died in the attack, the doctor told AP that 12 had been killed.
NATO had no immediate comment.
If the strike was by NATO warplanes, it would mark the third mistaken attack on rebel fighters in opposition territory since the airstrike campaign began several weeks ago.
On Tuesday, on the same road, NATO struck a convoy of 30 vehicles that belonged to Gaddafi’s forces, rebel spokesman Mohamed Ali said. The group of four-wheel-drive vehicles was destroyed.
Ali later said that Gaddafi’s forces continued to pour artillery fire into the heart of Misurata until after nightfall Thursday, killing at least 10 people and wounded more than 30.
“We expect that the casualties will be much higher,” he said via Skype. He added that NATO planes were heard in the sky throughout the day and were striking in and around the city, Libya’s third largest.
To the south of Misurata, where Gaddafi’s are stationed at the airport and the military air academy, rebel fighters were trying to push them farther from the city, Ali said.
The port is the lifeline to the besieged city, where more than 400 people have been killed in about two months of artillery fire, sniper fire and rocket attacks from Gaddafi’s forces.
Recent attacks by Gaddafi loyalists have focused on the port in an attempt to disrupt the flow of humanitarian aid and weapons being shuttled to the rebels by sea. If the port becomes too dangerous, the rebels will have no access to supplies, deeply impacting their ability to stave off Gaddafi’s offensive.
Rebel leaders therefore have begged for more NATO airstrikes, which they say will help save civilian lives. Even as some have denounced the civilian deaths from errant NATO strikes, the Western coalition has come under intense criticism in Misurata and the opposition east for not doing more to stop Gaddafi’s forces.
On Thursday, rebel leaders declined to fault NATO for the strike on the port road.
“If it was NATO, it means our boys are completely wrong to go there,” Ali said, via Skype. “They were told not to go there by commanders, and we accept responsibility for this mistake. No one in Misurata is blaming NATO for what happened.”
Shelling from Gaddafi forces continued on Thursday in the southern area of Karzaz. On Wednesday, at least 13 people were killed, including the fighters killed in the apparent NATO attack.
Aiman Abu Shahma, a doctor in Misurata, said a Nigerian migrant worker and a Libyan civilian were among those killed in shelling and rocket fire Thursday. The barrage included Grad 122mm rockets, which reached the city center and landed just a few hundred yards from the Misurata Polyclinic, Abu Shahma said via Skype.
“Every day he is shelling us with rockets, but rockets cannot occupy our city,” he said.
Abu Shahma said he was happy with NATO and that he hoped the alliance would carry out more airstrikes, notably on Gaddafi’s Bab al-Aziziyah compound in Tripoli.
Correspondent Simon Denyer in Tripoli, Libya, contributed to this report.