A mortar attack by forces loyal to Libyan leader Muammar Gaddafi killed at least one person and wounded several others in Misurata early on Sunday morning.
Two mortar shells hit a building that had previously been used to treat the wounded, but patients and medical staff had left a few days ago, a resident told the Reuters news agency.
Ambulances carrying an unknown number of wounded were arriving at a new building being used as a makeshift hospital, the witness said after the shelling.
A man named Ayman who said he was a doctor in Misurata told BBC Radio that the clinic was overwhelmed.
"We have one killed, three in the operating room now, one with an amputated leg, we have one in ICU (intensive care) because of shell fragments in his chest and we have six wounded with different wounds and they are waiting for an operation but we have only three operating rooms," he said.
Misurata is Libya's third-largest city and the last major hold out for opposition fighters who have been resisting bombardment and attacks from Gaddafi's troops for weeks.
'Accidental' strike
On Sunday, Libya's opposition fighters also admitted that a NATO air strike near Brega that killed at least 13 opposition fighters on Friday had been caused by an opposition fighter firing an anti-aircraft gun into the sky during a NATO sortie in the area. The man had apparently been firing in celebration, since opposition forces had advanced into Brega, and an opposition spokesman apologized for the incident.
"The leadership is working on preventing a reoccurrence," spokesman Abdulhafiz Ghoga said, adding that Brega "is fully under the control of the opposition".
Brega has been the scene of intense exchanges over the past few days when pro-Gaddafi forces returned after being driven out by opposition forces.
But it has been unclear since Thursday who actually held the town, with anti-Gaddafi forces regrouping in Ajdabiya, about 80km the east.
Earlier, a civilian official said the dead civilians were an ambulance driver and three medical students from Libya's second city of Benghazi.
They had been part of an opposition convoy of five or six vehicles, said Issa Khamis, liaison officer for the opposition's transitional government in the town of Ajdabiya, east of Brega.
Friday's air strike came as opposition fighters shot tracer fire into the air to celebrate the entry of an advance column into Brega.
"It was a mistake" by the opposition fighters, Khamis said. "The aircraft thought they were coming under attack and fired on the convoy."
NATO concerns
A spokeswoman for NATO, which leads the coalition enforcing the no-fly zone over Libya and protecting civilians from attack, said it was checking details of the incident.
"We are looking into these reports. We are always concerned by reports of civilian casualties. NATO's mission is to protect civilians and civilian areas from the threat of attack," said Oana Lungescu, adding that no formal investigation has been launched.
Speaking to Al Jazeera earlier, Mustafa Gheriani, a Transitional National Council spokesman, said the loss of lives on Friday was very much regretted.
"However, we understand that collateral damage may also take place and we do accept it, because we look at the big picture which saving more lives.
The Libyan government, meanwhile, has produced a video said to show civilians, including women and children, in a Brega hospital. They are believed to have been wounded as they tried to escape the air strikes.
Doctors say more than 240 people have been killed and over 1,000 wounded in Misurata in the last month alone, as a counter-offensive by Gaddafi's troops raised the number of casualties.
On Saturday, the first three Swedish fighter jets landed in Italy as the Nordic country joins the NATO-led no-fly zone operation over Libya. Five more will leave for the mission on Sunday, Rickard Wissman, an air force spokesman, said.
PHOTO CAPTION
Revolution fighters pray at the graves of around 10 fellow fighters who were killed in a coalition airstrike on a group of vehicles along the road between Ajdabiyah and Brega April 2, 2011.
Al-Jazeera