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. Urban targets 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. Unmanned US drones are already used to target militants along the Pakistan-Afghanistan border. US Defence Secretary Robert Gates has said two Predator drones were being made available for Nato use, but insisted there were still no plans to send in US ground troops. The Pentagon said the first drone mission took place on Thursday but turned back because of bad weather. Libya's deputy foreign minister warned that the drones 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." 'Morale boost' Meanwhile, US Senator John McCain has arrived on a visit to the rebel stronghold of Benghazi. A crowd of about 50 people greeted him at the courthouse in Benghazi, chanting, "The nasty Gaddafi has left and McCain came", the AFP news agency reports. Senator McCain, a former Republican presidential candidate, is the highest-ranking US official to visit the rebel-held east of the country since the uprising began two months ago. Mr McCain said the rebels needed "a lot of help" and wanted the world to follow France and Italy in formally recognising them as Libya's leaders. "I just came from the hospital where I saw a number of people who are badly wounded and dying," he said. "That frankly puts a face on it that argues maybe we should be doing everything we can to help these people, and maybe we're not and they're dying." The BBC's Peter Biles in Benghazi says the rebels are looking for every ounce of support they can get, so Mr McCain's visit will have been a huge morale boost. 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. Source: BBC