Misrata has come under attack by Libyan government forces attempting to retake the besieged city.
Three people were reportedly killed as missiles slammed into the city's port, a lifeline for those seeking to escape to the rebel stronghold Benghazi.
Nato is enforcing a UN resolution to protect civilians in Libya amid a two-month revolt inspired by other uprisings in the Arab world.
A Nato air strike earlier damaged Col Gaddafi's compound in Tripoli.
That attack followed big explosions at the Libyan leader's sprawling Bab al-Azizia compound early on Monday, which government spokesman Moussa Ibrahim called "an attempt to assassinate the leader and other political leaders of this country".
The Tripoli strikes have sparked angry criticism from Russian Prime Minister Vladimir Putin, who said the Western coalition had no mandate to kill Col Gaddafi.
Deadly shelling
The rebel-held city of Misrata has been besieged by government forces for two months, during which parts of the city have had neither electricity nor water.
Continued sniper fire, street clashes and shelling have prevented people from venturing outside of their homes to get food and medicine.
Human rights groups say more than 1,000 people have been killed in the fighting and many more wounded. Ships have been ferrying the wounded to hospitals in the rebel stronghold, Benghazi.
At the weekend, Col Gaddafi's regime claimed to have suspended operations in the city, and its forces withdrew in places.
Since then, however, the deadly shelling has continued, with Misrata surrounded on three sides and seeing some of the worst violence since the siege began.
"It was horrific, like a scene from World War II," said Misrata resident Saddoun el-Misurati as rockets began falling on the port on Tuesday.
He told the Associated Press he had been waiting with his mother for a boat to evacuate them from the port with hundreds of residents and migrant African workers. The crowds had scattered for cover when the shelling started, he added.
Libya's government denies it has been indiscriminately shelling civilian areas.
RAF fighter aircraft "successfully attacked" three armoured personnel carriers near Misrata over the weekend, Prime Minister David Cameron's office said, but warned UK forces "must prepare for the long haul" in Libya.
UK Foreign Secretary William Hague said the UK was taking steps to evacuate 5,000 people from the city, as well as providing much-needed food, water and medicine supplies.
"The regime must grant unfettered humanitarian access, not just broken promises that puts aid workers and civilians at risk," Mr Hague told MPs on Tuesday.
Mr Hague said Nato's 1,500 strike sorties over Libya had "seriously degraded Gaddafi's military assets and prevented widespread massacres planned by Gaddafi's forces.
"They remain unable to enter Benghazi and it is highly likely that without these efforts Misrata would have fallen, with terrible consequences for that city's brave inhabitants."
The UK has for weeks called for more commitment from Nato members to the operation in Libya, but on Monday Russia said the Western military intervention risked fanning a series of civil wars across the Middle East.
Italy, France and Britain last week agreed to send military advisers to assist the Benghazi-based rebel Transitional National Council (TNC) in its battle against pro-Gaddafi forces.
The US launched its first Predator drone strikes pro-Gaddafi military positions over the weekend, which the Libyan government denounced as crimes against humanity.
The UNHCR estimates some 30,000 civilians have fled their homes in Libya's Western Mountain region and crossed to southern Tunisia in the past three weeks, leaving some towns virtually deserted.
On Tuesday the first UNHCR humanitarian plane landed in Benghazi carrying relief assistance including hospital tents, kitchen sets as well as plastic sheets for shelter.
Meanwhile, it has emerged Libya has been taking advantage of a loophole in United Nations sanctions against Col Gaddafi's government by importing gasoline from the Italian refiner, Saras.
The shipment is legal because the buyer, Libya's General National Maritime Transport Company (GNMTC), is not on a UN blacklist.