The United States has started using armed drones against Muammar Gaddafi's troops, who battled rebels at close quarters on the streets of Misrata, despite Western threats to step up a month-old air war.
Rebels welcomed the deployment of US unmanned aircraft and said they hoped the move would protect civilians.
Arizona Senator John McCain flew to Benghazi to meet with rebel leaders on Friday, telling the press that the rebels are his heros.
Doctors at the hospital in Misrata, the rebels' last major bastion in the West of the country, said nine insurgents were killed in fighting on Thursday.
Defense Secretary Robert Gates told a Washington news conference President Barack Obama had authorized the use of Predator drones and they were already in operation.
General James Cartwright, vice chairman of the US Joint Chiefs of Staff, said the first two Predators 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 weapon in Pakistan and other areas where US forces have no troops on the ground. They can stay aloft nearly perpetually without being noticed from the ground and hit targets with missiles, with no risk to crew.
"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.
Secretary of State Hillary Clinton said Gaddafi's forces were carrying out "vicious attacks" on Misrata and might have used cluster bombs against civilians.
Hundreds of people are believed to have died in Misrata during its siege. At the hospital, ambulances raced in carrying wounded fighters. Doctors said that four of the nine rebels killed died in a fierce battle around the Tripoli Street thoroughfare.
Rebel Salman al-Mabrouk said a group came under fire when they tried to enter a building occupied by pro-Gaddafi snipers.
"We suddenly discovered they had surrounded us on all sides and they opened fire. It seems many government soldiers were inside buildings around the one where we tried to get into."
Rebel fighters voiced frustration with an international military operation they see as too cautious.
"NATO has been inefficient in Misrata. NATO has completely failed to change things on the ground," rebel spokesman Abdelsalam said.
Food and medical supplies were running out and there were long queues for petrol. Electricity was cut so residents depended on generators. Thousands of stranded foreign migrant workers awaited rescue in the port area.
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 organization 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.
UN Secretary-General Ban Ki-moon urged Libyan authorities to stop fighting, saying during a visit to Moscow that the priority of the United Nations was to secure a ceasefire.
French President Nicolas Sarkozy, who has spearheaded U.N.-backed NATO intervention, pledged stronger military action at his first meeting with the leader of the opposition Libyan National Council, Mustafa Abdel Jalil, on Wednesday.
The French Defense 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 organized 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 told reporters the government welcomed ships coming to Misrata to pick up foreign workers. However, it would not accept international humanitarian aid arriving "with military cover."