Libyan rebel fighter holding a rocket-propelled grenade (RPG) Libya accused Britain of bombing Libya's biggest oil field as Washington rebuffed a letter from Moamer Kadhafi to President Barack Obama, reiterating calls for the strongman to step down.

As the war between government and rebel forces dug deeper into a stalemate and amid mounting alarm at the plight of civilians in besieged Misrata, Western powers were Thursday throwing their energies into negotiating a solution. NATO, acting under a UN mandate to protect civilians, says it is hamstrung in targeting Kadhafi's forces as they are using human shields but vowed to do everything to protect the population in Misrata, where UN Secretary General Ban Ki-moon said conditions were "grave."  Libyan deputy foreign minister Khaled Kaim told a news conference in Tripoli that British warplanes had bombed Libya's biggest oil field at Al-Sarir in the southeast.The bombing, he said, had damaged the pipeline linking Al-Sarir and Tobruk, which is under rebel control.
"British fighter bombers raided the Al-Sarir oil field, killing three guards at the site and wounding other people working at the field," Kaim said.
There was no immediate comment from Britain's defence ministry or from NATO, which is coordinating the air strikes. A tanker left Tobruk on Wednesday carrying the first consignment of oil since the rebel government won recognition from some countries.It had docked a day earlier in order to load the consignment of Libyan crude worth up to 100 million dollars for export, the first since the international coalition air strikes began on March 19 and destined to finance the rebel fight against Kadhafi's forces.
The White House acknowledged receiving a letter from Kadhafi, in which, according to excerpts widely published in the US media, the Libyan strongman urged Obama to end the air strikes. The official Libyan news agency JANA had said Kadhafi sent the letter to Obama following the withdrawal of US warplanes from front line missions in the coalition air operation.Asked about the letter, US Secretary of State Hillary Clinton was dismissive. "I don't think there is any mystery about what is expected from Mr. Kadhafi at this time," she told reporters. "I think that Mr. Kadhafi knows what he must do, there needs to be a ceasefire, his forces need to withdraw from the cities that they have forcibly taken at great violence and human cost. "The sooner that occurs and the bloodshed ends, the better it will be for everyone," the chief US diplomat said. Speaking alongside visiting Italian Foreign Minister Franco Frattini, she added: "There needs to be a decision made about his departure from power and as (the) foreign minister said his departure from Libya."
The United Nations called for a halt to hostilities around Misrata saying several weeks of "heavy shelling" by Kadhafi's forces had killed or wounded hundreds.
UN chief Ban made a new "urgent call for an immediate cessation of the indiscriminate use of military force against the civilian population," his spokesman said. "Conditions in Misrata are especially grave, with reports of the use of heavy weapons to attack the city, where the population is trapped and unable, as a result of heavy shelling that has continued over several weeks, to receive basic supplies, including clean water, food, and medicines," he said.

Libya's third largest city, about 215 kilometres (130 miles) east of Tripoli, has seen fighting for more than 40 days since the start of the uprising against Kadhafi.
NATO, accused of mission failure by Libyan rebels, said government troops are using civilians as human shields in Misrata, making its mission more precarious. Focus: Family awaits word of missing Libyan in bloody video. Rear Admiral Russell Harding, NATO's deputy commander of operations, told journalists that "NATO forces have been particularly careful to avoid injury to civilians who are in close proximity to the fighting, often precisely because of the tactics of government forces." Kadhafi loyalists "have increasingly shifted to non-conventional tactics, blending in with road traffic and using civilian life as a shield for their advance," he said at the base in Naples overseeing operations. The mandate to protect the civilian population was more challenging because of the stipulations of the UN Security Council resolution approving the mission, which explicitly rules out the use of ground troops, he said.
The top commander of rebel forces, Abdelfatah Yunis, had earlier in the week accused NATO-led aircraft of doing nothing while loyalist forces kept up bombardment of civilians in Misrata.
On the ground, the poorly performing rebels are making no headway against Kadhafi's better-armed forces, and fighting has bogged down over the past week near Brega, a thinly populated desert town with a strategic oil depot.
The threat of NATO air strikes has kept the regime's fighters from advancing into the rebel city of Ajdabiya, but their mobile artillery can easily push back the insurgents -- as they have already shown several times this week. With the fighting at a point of stalemate, the US, France and Britain are reaching out to both the rebels and, indirectly, to officials in Kadhafi's regime, looking for a way to bring them together in talks.
Envoys from those countries were in the opposition stronghold of Benghazi holding talks with rebel leaders, and Turkey -- the only Muslim member of NATO -- was maintaining communication with Kadhafi's circle. But there was as yet no agreement on opening negotiations, with both sides imposing conditions.
The rebels were adamant that they would not speak with Kadhafi, his sons or his closest aides, opposition spokesman Shamsiddin Abdulmolah told AFP.
But deputy foreign minister Kaim said that while the Libyan regime was willing to look at reforms, no dialogue would happen until the rebels "lay down their arms."  And government spokesman Mussa Ibrahim said Monday that there was no way Kadhafi would resign.