Libya: Stalemate looms, warns Admiral Mike Mullen
The most senior US soldier, Adm Mike Mullen, has said the war in Libya is "moving towards stalemate", even though US and Nato air strikes have destroyed 30-40% of Libya's ground forces.
The US has authorised the use of armed, unmanned Predator drones over Libya to give "precision capabilities".
Libyan rebels have been battling Col Gaddafi's troops since February but have recently made little headway.
Adm Mullen also said there was no sign of al-Qaeda in the Libyan opposition.
Speaking to US troops in Iraq, he said radical groups might try to take advantage of the Libyan uprising, but added: "We're watchful of it, mindful of it and I just haven't seen much of it at all. In fact, I've seen no al-Qaeda representation there at all."
Last month, a Nato commander said US intelligence had picked up "flickers" of al-Qaeda activity among the rebels.
Meanwhile Senator John McCain has visited the rebel stronghold of Benghazi, the most senior US official to do so since the conflict began in February.
He called on all countries to recognise the rebels' Transitional National Council as "the legitimate voice of the Libyan people", and to offer weapons and training.
A fourth evacuation ship chartered by the International Organisation for Migration is on its way to rescue stranded migrant workers and wounded civilians from the besieged city of Misrata.
Col Gaddafi's forces have been pounding the city - the main rebel-held area in western Libya - for weeks.
Medics say more than 1,000 people have died since the fighting began, many falling victim to snipers.
The BBC has seen evidence that cluster bombs are being used by pro-Gaddafi forces in Misrata, a charge they have so far denied.
Government spokesman Moussa Ibrahim has warned the international community to stay out of Misrata, saying the government will "unleash hell" if foreign troops enter the city.
The US hopes the low-flying pilotless drones will be able to accurately target pro-Gaddafi units on the ground in built-up areas like Misrata, where there is a high risk of civilian casualties.
The Pentagon said the first drone mission took place on Thursday but turned back because of bad weather.
The BBC's Paul Adams in Washington said the use of drones - operated remotely from bases in the US - is both convenient and symbolic, allowing the White House to stick to its mantra of not putting boots on the ground in Libya.
But our correspondent adds that drones are not infallible, and that appalling mistakes have been made where they have been used in Pakistan, Yemen and Afghanistan.
The drone deployment has been welcomed by the rebels, but Libya's deputy foreign minister warned their use would increase civilian casualties and would not change the outcome of the conflict.
"They [drones] will kill more civilians and this is very sad," Khaled Khaim told the BBC. "It's for the Libyans to choose their destiny - not by sending more weapons or more air strikes, or more money and weapons to the rebels."
"I think what they are doing is undemocratic, illegitimate. I do hope that they will reverse their decision."
At a news conference in Benghazi, Mr McCain said the rebels should be offered "every appropriate means of assistance", including "command and control support, battlefield intelligence, training and weapons", AP reports.
Mr McCain said visiting the wounded in hospital had led him to question whether the world was doing enough to help.
"I would encourage every nation, especially the United States, to recognise the Transitional National Council as the legitimate voice of the Libyan people," he said.
"They have earned this right and Gaddafi has forfeited it by waging war on his own people."
On Thursday, Libyan rebels were reported to have seized control of a border post on the Tunisian border in a rare advance in the west.
Reports say about 100 government soldiers handed themselves in in Tunisia after intense fighting in the Western Mountains region.
Restrictions on journalists in remote areas of Libya mean it is hard to independently verify such reports.
The UN's refugee agency says more than 14,000 people have fled the Western Mountains into Tunisia in the past two weeks.