The top US military officer said air strikes had hobbled Libyan forces but the conflict was moving into "stalemate" as Muammar Gadafy's troops pressed on with their punishing siege of rebel Misrata.
Morocco said it was seeking a political solution to the crisis, after Moroccan officials met representatives of Col Gadafy and rebels this week.
Rebels welcomed US plans to deploy unmanned aircraft, typically operated remotely from the United States. But it emerged that bad weather had forced the first two drones sent to Libya to turn back.
"It's certainly moving towards a stalemate," said Admiral Mike Mullen, chairman of the US military's joint chiefs of staff, in Baghdad.
"At the same time we've attrited somewhere between 30 and 40 per cent of his main ground forces, his ground force capabilities. Those will continue to go away over time."
In Misrata, the only rebel-held major city in western Libya, rebels wrested control of a downtown office building which had been a base for Col Gadafy's snipers and other troops, after a furious two-week-long battle.
Shattered masonry, wrecked tanks and the incinerated corpse of a government soldier lay near the former insurance offices today amid buildings pockmarked by gunfire.
"They shot anything that moved," one fighter said of the Gadafy men driven out.
Rebels said they had captured several other central buildings from governent forces and the state of the battle did not appear to match claims by govermnment officials in Tripoli to control 80 per cent of Misrata.
Rebel fighters are fighting a block-by-block war of attrition with an enemy sometimes only yards (metres) away.
"Gadafy's fighters taunt us. If they are in a nearby building they yell at us at night to scare us. They call us rats," one rebel said.
Ambulance drivers on the rebel side accuse government forces of deliberately shooting at their vehicles. Government officials deny attacking civilians.
Hundreds of fighters and civilians have died in Misrata during the siege. Rebel fighters voice frustration with an international military operation they see as too cautious.
Food and medicine are running out and there are queues for petrol. Residents depend on generators for power and thousands of stranded foreign workers await rescue in the port area.
General James Cartwright, vice chairman of the US Joint Chiefs of Staff, said the first two Predator drones were sent to Libya yesterday but had to turn back because of bad weather.
The United States planned to maintain two patrols of armed Predators above Libya at any given time, Cartwright said.
The drones have proven a potent but controversial weapon in Pakistan and other areas where US forces have no troops on the ground. They can fly without being noticed from the ground and hit targets with missiles with no risk to crew. However, they have killed many civilians by mistake in Pakistan.
"There's no doubt that will help protect civilians and we welcome that step from the American administration," rebel spokesman Abdel Hafiz Ghoga said on Al Jazeera television.
The Obama administration has been anxious not to spearhead Nato's Libya campaign. It has instead let British and French planes do the bombing and has not deployed low-flying ground attack aircraft, unique to US forces, which military analysts say would be most effective against Gadafy's troops.
Analysts said the drones were a way of appeasing French and British calls for more US help but were far from being a "silver bullet" to tilt the conflict against Col Gadafy.
Libyan state television said nine people had been killed overnight by Nato bombardment of the city of Sirte, Col Gadafy's home town, including employees of the state water utility.
US secretary of state Hillary Clinton said Gadafy's forces were carrying out "vicious attacks" on Misrata and might have used cluster bombs against civilians. The United States, like Libya, has not joined a convention banning such weapons.
Republican US Senator John McCain went to the eastern rebel stronghold of Benghazi today for talks with rebel leaders. He said they needed more help to overthrow Col Gadafy and "get this thing over with".
An official Moroccan source said Rabat was working for a solution which would "respond to the aspirations of the Libyan people and ensure a return of stability which is extremely important for the Mediterranean and sub-Saharan Africa".
France, Libya and Britain plan to dispatch up to small teams to help rebels improve organisation and communications.
Tripoli denounced such moves and some commentators warned of "mission creep" after assurances by Western leaders that they would not put combat troops in Libya.
Russia said the sending of advisers exceeded the UN Security Council mandate to protect civilians.
Libya urged rebels on Thursday to sit down to peace talks but said it was arming and training civilians to confront any possible ground attack by Nato forces.
In Tripoli, Ibrahim said the government welcomed ships coming to Misrata to pick up foreign workers. However, it would not accept international humanitarian aid arriving "with military cover".
Coastguard and port officials said only two ships had docked at Tripoli's once-bustling port since mid-March and a Nato-enforced arms embargo was strangling trade and stopping fishermen from putting to sea.
Fighting continued in the rebel-held east of the country. Gaddafi's forces fired shells towards the town of Ajdabiyah on Thursday, seeking to dislodge anti-government fighters from their main outpost before the rebel stronghold of Benghazi.
Rebels showed off weapons captured from Muammar Gadafy's fleeing soldiers today, a day after seizing a remote border crossing near the Tunisian town of Dehiba.
Reuters