The Libyan government has promised aid workers access to areas under its control, according to the UN.
UN officials say the agreement allows humanitarian workers to set themselves up in the capital, Tripoli, and to come and go freely into Libya.
Meanwhile, about 1,000 people who were evacuated from Misrata have arrived in the rebel base of Benghazi.
Pro-Gaddafi forces have been pounding the city for days and hundreds of people are thought to have been killed.Those evacuated were brought out on a ship chartered by the aid agency, the International Organisation for Migration, which says thousands more are waiting to be rescued.
Rebel forces on Monday said they were making ground in Misrata, but that pro-Gaddafi forces were continuing to press the rebel-held town of Ajdabiya in the east.
'Vehicles and equipment'
The CA's Barbara Plett says that the Libyan authorities have promised aid workers and their equipment access to all areas under their control.
But the besieged city of Misrata is not within their control - it is held by the rebels, she adds.
For aid workers to help the people there the fighting would have to stop but, according to the UN, the government has not committed itself to a ceasefire.
The city has been heavily bombarded by pro-Gaddafi forces in recent days, and is running short of basic food and medical supplies.
The head of Nato in Libya, Lieutenant-General Charles Bouchard, said Col Gaddafi's forces had employed what he called underhand and immoral tactics in their seven-week drive to dislodge the rebels from the city.
"Inside the city it's a very difficult tough situation.
"Gaddafi forces have taken their uniforms off, they're hiding on rooftops of mosques, hospitals, schools, that's where their heavy equipment is positioned, near mosques, near schools, and they're shielding themselves with women and children.
"So when people ask me why aren't you doing something, well I'm not going to lower to his level. I'm not going to do the kind of warfare that he's doing. My job's to help the population," he said, in an interview with the Canadian Broadcasting Corporation.
'Limited time'
Meanwhile, the Ionian Spirit arrived in the eastern city of Benghazi carrying nearly 1,000 foreign workers and wounded Libyans from the city of Misrata.
Some of those who reached Benghazi on Monday evening accused Col Gaddafi's forces of firing indiscriminately.
The IOM has said that thousands more people in Misrata were waiting to be rescued from what it described as an increasingly perilous situation.
"We wanted to be able to take more people out but it was not possible," said Jeremy Haslam, who led the IOM rescue mission."Although the exchange of fire subsided while we were boarding... we had a very limited time to get the migrants and Libyans on board the ship and then leave."
The UK has pledged to pay for the evacuation of 5,000 people from Misrata.
A rebel spokesman in Misrata told Reuters on Monday they had "made progress" in the city and were "controlling some areas surrounding Tripoli Street".
However, he said rebel fighters were facing snipers and troops armed with rocket-propelled grenades.
Despite last month's UN resolution authorising air strikes to protect civilians in Libya, rebels have been unable to retain territory during fighting along the coastal towns of eastern Libya.
Col Gaddafi is defying international pressure to step down despite the revolt against his 41-year rule that began in Benghazi in February.
