|The western city of Misurata has borne the brunt of bombardment by Gaddafi's forces for almost six weeks
Pro-democracy fighters are battling Muammar Gaddafi's, the Libyan leader, forces on the country's western border, while fighting continues in the besieged rebel-held city of Misurata.
A rebel spokesman told Al Jazeera on Wednesday that heavy shelling had prevented ships from docking in Misurata to deliver humanitarian supplies and evacuate the wounded.
"The attacks have prevented a number of ships seeking to enter the port to evacuate the wounded from Misurata," Ahmed Hassan, the spokesman, said.
"Other ships are also waiting to unload shipments from humanitarian agencies."
The rebels said the Libyan army had withdrawn from central Misurata but that fierce fighting was still ongoing for control of the city's port.
"Gaddafi's forces retreated from the port area where they were positioned yesterday after air strikes
by the NATO forces," an opposition spokesman, called Reda, told the Reuters news agency.
"The strikes completely destroyed 37 military vehicles.
"Gaddafi's forces this morning started bombarding an area about 10km north of the city. It is known as the Steel area.
"The bombardment is still going on. They are using Grad missiles ... Warplanes are flying over Misurata's outskirts but I don't hear any sound of strikes."
While government forces pulled out of Misurata over the weekend under pressure from NATO air attacks, they have since unleashed a heavy bombardment on what is Libya's third-largest city.
"Another development which is rather disturbing is that there has been shelling in the docks area, which is the lifeline to opposition forces and indeed the civilian population here," Al Jazeera's Andrew Simmons reported from Misurata on Wednesday.
US Senator John McCain, who recently visited Benghazi, advocates the use of air power in Libya [ Al Jazeera]
"Reports from an opposition spokesperson say pro-Gaddafi troops are intent to cut the main road from city to the port. If the lifeline is cut, then everyone is in a desperate situation.
"Though Gaddafi troops have left the city, everybody is under threat of shellfire. The whole city centre has been destroyed and needs to be rebuilt."
With fears of a military stalemate looming, John McCain, the most prominent American politician to visit Libya since the start of the conflict, has called for the use of more air power to prevent a humanitarian crisis in Libya.
"If you are really in this to prevent a humanitarian disaster, the only way you can prevent it is to have Gaddafi not able to inflict it," he told Al Jazeera.
The senator's comments came amid reports that thousands of migrant workers have been left stranded in Misurata, waiting for relief organisations to ship them to safety.
One ship sent to evacuate 1,000 migrants from the city docked on Wednesday in the port after spending a night offshore awaiting a lull in shelling, a Swiss aid agency said.
Caught in the crossfire
International Organisation for Migration (IOM), based in Geneva, said at least one migrant from Niger was reported killed and up to 20 others wounded in the bombardment of Misurata a day earlier.
At least 1,500 migrants, most from sub-Saharan Africa including Niger and Chad, have gathered at the port about 10km to 12km east of the city awaiting rescue, IOM said.
It said tens of thousands of migrants have fled the conflict in Libya into northern Niger and Chad, and are in need of urgent support.
Against this backdrop of rising casualties and civilian suffering, the Obama administration has eased its sanctions against Libya to allow for the sale of oil controlled by the rebels.
The move will allow Libya's opposition forces to use the income from oil sales to buy weapons and other supplies.
The US has also ordered the expenditure of up to $25m in surplus government goods to support Libyan opposition groups and protect civilians threatened by Gaddafi forces.
Italy and France have called on the international community to stop shipping oil products to the Libyan government and urged market operators not to buy its crude oil.
Libya, in turn, has urged Russia to call an emergency meeting of the UN Security Council to discuss what it called "Western aggression".