The United States has launched a diplomatic offensive against Libya among African nations as Tripoli accused NATO of a "massacre" of 85 villagers in air strikes in support of rebels.

American diplomats are visiting several African countries as part of efforts to urge leaders to press Libyan strongman Moamer Gaddafi to leave power immediately, officials in Washington said on Tuesday.

Several African states, having benefited financially from Gaddafi's policies, have been reluctant to call for him to step down, and have criticised the NATO-led military campaign in Libya.

Gene Cretz, the US ambassador who left Tripoli before Gaddafi launched his bloody crackdown on the opposition in February, and Principal Deputy Assistant Secretary of State Donald Yamamoto arrived Monday in Addis Ababa, headquarters of the African Union, State Department deputy spokesman Mark Toner said.

They "are in Africa to meet with African Union members to discuss the crisis in Libya and the need for Gaddafi to relinquish power now," he told AFP.

They also met Mahmud Jibril, leader of Libya's opposition National Transitional Council (NTC) who was visiting Ethiopia.

Gaddafi, meanwhile, said world powers would be held responsible for the "ugly massacre committed by NATO" on the village of Majer where 85 people were killed, Libya's official JANA news agency reported.

Majer, 10 kilometres (six miles) south of Zliten 120 kilometres east of Tripoli, was attacked late on Monday to try to help rebel fighters enter the government-held city from the south, government spokesman Mussa Ibrahim said.

"After the first three bombs dropped at around 11:00 pm (2100 GMT) on Monday, many residents of the area ran to the bombed houses to try to save their loved ones. Three more bombs struck," he told reporters on an organised visit.

Thirty-three children, 32 women and 20 men from 12 families were killed in the "massacre," Mussa said.

Reporters attended the funerals of victims and saw 28 bodies buried at the local cemetery where hundreds of people vented their anger against NATO, "the spies and the traitors," an AFP correspondent said.

In the hospital morgue, 30 bodies -- including two children and one woman -- were shown along with other bodies which had been torn apart.

NATO, which launched its air campaign at the end of March under UN resolutions to protect civilians against Gaddafi's forces, insisted the raids were "legitimate" and said it had no evidence of civilian deaths.

"We do not have evidence of civilian casualties at this stage," the NATO spokesman for the alliance's Libya campaign, Colonel Roland Lavoie, said from his Naples headquarters.

NATO raids south of Zliten were against two former farms used for military purposes by Gaddafi forces, he said. "This was a military facility clearly... NATO takes extreme precaution not to harm innocent civilians living or working nearby."

JANA said Gaddafi sent a message to the heads of state of UN Security Council members saying "they should bear responsibility for the ugly massacre carried out by NATO in Majer."

He was quoted as saying "there has never been such a massacre throughout the history of wars."

Rebels fighting around Zliten said on Monday they were running low on ammunition as they struggled to hold off an assault by loyalists.

The rebels, advancing from the nearby port city of Misrata, punched into the centre of Zliten a week ago. But they later pulled back to the edge of the city of 200,000 inhabitants.

Elsewhere on the battlefield, at least two rebels were killed in fighting on Tuesday around the oil town of Brega in eastern Libya, a rebel spokesman said.

NATO said it had hit nine targets in the Brega area on Tuesday, listing them as a military facility, one tank, four armed vehicles and three multiple rocket launchers.

The alliance also said it hit nine anti-aircraft systems and eight surface-to-air missile systems in Tripoli.

On the diplomatic front, France said new EU sanctions were imposed on Gaddafi's regime. Canada and Denmark, which have both recognised the NTC, also expelled pro-Gaddafi diplomats, officials said.

And the United States said it has transferred the Libyan embassy in Washington to the NTC, which it has recognised as Libya's de facto government.

On Wednesday, JANA quoted Libyan Deputy Foreign Minister Khaled Kaaim as saying British Prime Minister David Cameron should step down as he had "lost all legitimacy" because of the riots shaking Britain since Saturday.