The top US military officer said air strikes had hobbled Libyan forces but the conflict was moving into "stalemate" as Muammar Gaddafi's troops pressed on with their punishing siege of rebel Misrata.
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, addressing US troops during a visit to Baghdad.
"At the same time we've attrited somewhere between 30 and 40 percent 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 large downtown office building which had been a base for Gaddafi'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 on Friday amid buildings pockmarked by gunfire.
"They shot anything that moved," one fighter said of the Gaddafi men driven out.
Rebels said they had captured several other central buildings from government forces and the state of the battle did not appear to match claims by government officials in Tripoli to control 80 percent of Misrata.
Rebel fighters are fighting a block-by-block war of attrition with an enemy sometimes only yards (metres) away.
"Gaddafi'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.
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 medical supplies are running out and there are long queues for petrol. Residents depend on generators for power and thousands of stranded foreign workers are awaiting 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 on Thursday 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, raising questions about their use.
"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 Gaddafi's troops.
Libyan state television said nine people had been killed overnight by NATO bombardment of the city of Sirte, Gaddafi's home town, including employees of the state water utility.
US Secretary of State Hillary Clinton said Gaddafi'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 Senator John McCain, who favours a greater US involvement in Libya, arrived in the eastern rebel stronghold of Benghazi on Friday for talks with rebel leaders.
France said it would send up to 10 military advisers to Libya and Britain plans to dispatch up to a dozen officers to help rebels improve organisation and communications. Italy is considering sending a small military training team.
Tripoli denounced such moves and some commentators warned of "mission creep" after assurances by Western leaders that they would not put "boots on the ground" in Libya.
Russia said the sending of advisers exceeded the UN Security Council mandate to protect civilians.
"We are not happy about the latest events in Libya, which are pulling the international community into a conflict on the ground. This may have unpredictable consequences," Russian Foreign Minister Sergei Lavrov said.
The French Defence Ministry said on Thursday it had increased the number of its air sorties in the past week to 41 from an average of 30 since the start of the operation.
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.
"Many cities have organised themselves into squads to fight any possible NATO invasion," government spokesman Mussa Ibrahim said, adding that authorities were handing out rifles and guns.
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".
Fighting continues 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.