Tribes loyal to Libyan leader Muammar Gaddafi have said that if the army cannot drive rebels from the besieged port city of Misrata, they will, a senior official says. Deputy Foreign Minister Khaled Kaim said the army had tried to keep civilian casualties low but the tribes would not show the same restraint. Colonel Gaddafi's forces have been pounding Misrata for weeks. Meanwhile, Nato forces carried out more air strikes on the capital, Tripoli. The Libyan government says three people were killed by the strikes. Journalists were shown a concrete bunker near Col Gaddafi's Bab al-Azizia compound that received two hits early on Saturday. Ultimatum Aid organisations say Misrata - the main rebel-held area in western Libya - faces a humanitarian crisis after weeks of fighting. Human rights groups say more than 1,000 people there have died. The BBC's Jeremy Bowen reports from Tripoli that the regime says the reason Col Gaddafi has remained relatively secure in the west of Libya is that the principal tribes - which wield a lot of power and influence in the country - are on his side. However, the government has previously used the prospect of tribal civil war as a warning against rebel leaders and Nato intervention, and it may well be that the minister was making more of a threat than expressing the reality of what is going to occur, our correspondent says. The regime is feeling increasingly isolated and is hoping for some kind of a diplomatic solution, he adds. The comments came in a meeting between tribal leaders and the military in the area of Misrata still controlled by the government, Mr Kaim said. He said the tribes were angry that people's lives had been disrupted by weeks of fighting that had cut the main coastal road and stopped trade in the city. Tribal leaders say the seaport is for all Libyans and not just the rebels, Mr Kaim said. 'Surgical' tactics In normal times Misrata is a major commercial centre and its port is second only to Tripoli. "Now there is an ultimatum before the Libyan army. If they can't resolve the problem in Misrata then the people from the region... will move in," he told reporters. He said the tribes would first try to persuade the rebels to lay down their arms, but if that failed they would move in. The army would stay where it was, he added. "The tactic of the army is to have a surgical solution but with the (Nato) air strikes it doesn't work," Mr Kaim said. The comments came amid reports of setbacks for pro-Gaddafi forces. A wounded government soldier captured by rebels told Reuters news agency that Col Gaddafi's forces had been told to withdraw from Misrata on Friday, and rebels captured an eight-story insurance building from which dozens of government snipers had been operating. Meanwhile, a fourth evacuation ship chartered by the International Organisation for Migration is planning to rescue more stranded migrant workers and wounded civilians from the besieged city. Earlier, the most senior US soldier, Adm Mike Mullen, said the war in Libya was "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". A popular revolt against Col Gaddafi - inspired by similar uprisings in Egypt and Tunisia - began in February and a UN mandate later sanctioned air strikes against Libyan state forces to protect civilians. Nato took control of the operation on 31 March. Source: BBC