UK Defence Secretary Liam Fox has welcomed Washington's sanctioning of the deployment of armed Predator drones to attack ground targets in Libya.

Speaking at a news conference at the Pentagon with his US counterpart Robert Gates, Dr Fox said it was clear that the Libyan regime was "on the back foot".

"We've seen some momentum gained in the last few days," said Fox, standing next to Gates outside the Pentagon.

"We've seen some progress made in Misrata. And it's very clear that the regime is on the back foot," he told reporters.

He also decried Libya's use of young mercenary soldiers on the front lines, saying it was a "sign of desperation from a regime that they resort to these sort of tactics".

The UN resolution approving military intervention focuses on protecting civilians against the regime's forces, but US and European leaders have made clear they want to see an end to Gaddafi's rule.

"The sooner that Colonel Gaddafi recognizes that the game is up, either today or shortly, the better. He is a liability for his people and his country, and the sooner that he gets this message the better," Fox said.

The besieged seaside city of Misrata has been the scene of deadly battles as rebels have sought to oust Gaddafi, who has been in power for more than four decades.

Hundreds of people have been killed in the battle for the city, which has been under siege by Gaddafi's forces for seven weeks. The city's population of about 450,000 makes it the country's third-largest.

"As fighting continues to rage in Misrata, the families recently evacuated by boats to Tobruk from the embattled city describe a catastrophic situation with many having lived in fear of indiscriminate shelling," the UN High Commissioner for Refugees said Tuesday.

"Many houses and buildings have been destroyed and some families had to move several times."

The UN refugee agency said parts of the city have lacked electricity and water and people have been hiding in their homes for two months "before seizing the opportunity of a lull in fighting to get to the harbor and board a boat."

"Sniper fire, street clashes and shelling have prevented people from venturing outside of their homes to get food and medicine," the agency said.

"In some neighborhoods in Misrata, pregnant women gave birth in their homes as it would have been too dangerous to make the trip to the hospital."

The agency also said that, in recent weeks, some 30,000 Libyan civilians have fled their homes in the western mountain region for Tunisia.