Muammar Gaddafi's forces have shelled the besieged rebel frontline city of Misrata, hitting residential areas and wounding four people, a doctor said.
Nato rejected a claim by another doctor in the city that one of its air strikes killed 12 rebels in a friendly fire incident a day earlier.
The alliance, which is enforcing a no-fly zone to protect civilians, denied its planes bombed a building the rebels were said to be occupying.
Gaddafi's best trained forces are battling fiercely to try to uproot rebel fighters from their only major stronghold in the western half of Libya, which is home to the government's power centres and the capital, Tripoli.
A doctor working in the city, Hassan Malitan, recounted details of what he believed was a Nato air strike on Wednesday on rebels holed up in a building about three miles east of the port. He believed the attack was a mistake but insisted it was caused by Nato aircraft, adding that the strike came moments after he and another doctor visited the building.
"We drove about 200 metres and we heard a huge explosion that shook the earth," Dr Malitan said. He said he looked back and saw smoke rising from where they had just sat with the men. As he and the other doctor began slowly driving back toward the building, a second missile crashed into it, Dr Malitan said.
Dr Malitan said he was surprised to see rebels so far east, and said they assured him that they had been in contact with Nato forces about their location.
Meanwhile, the military head of Libya's rebel forces has urged Nato to find more attack helicopters to counter Colonel Muammar Gaddafi's tanks.
General Abdul Fattah Younis, who left the regime to join the uprising and now has a four-million-dollar bounty on his head, was in Brussels for talks at Nato and EU headquarters.
He voiced fears that Col Gaddafi might have access to chemical weapons in the form of mustard gas, and that he would fight "to the bitter end" and commit suicide if necessary rather than concede defeat.