The United States and European Union turned up the heat on Libya, as fresh fighting erupted Wednesday along rebel lines at the oil town of Brega and state television showed footage of a son of Libyan leader Muammar Gaddafi, days after rebels reported him dead.

US diplomats are visiting several African countries to urge leaders to press Gaddafi to leave power immediately, officials in Washington said, while the EU slapped new economic sanctions on Gaddafi's regime.

Libyan television showed footage of Gaddafi's youngest son, Khamis, supposedly visiting victims of NATO raids on Tuesday. But a rebel spokesman insisted that the 28-year-old, a feared military commander, was dead.

The date of the recording could not be confirmed. If genuine, it would be the first time Khamis had been seen in public since Friday, when rebels said a NATO strike on the western town of Zliten killed 32 people, including Khamis.

Gaddafi spokesman Mussa Ibrahim said at the time the claim was untrue, claiming it was "dirty lies to cover the murder of civilians" in Zliten.

Khamis trained at a Russian military academy and commands the eponymous and much-feared Khamis Brigade, one of the regime's toughest fighting units.

Reacting to the television report, rebel spokesman Abudulah Kabir said in the city of Misrata that the insurgents were sure Khamis was dead.

"The military council has confirmed he has died; we are sure," he said.

Fresh fighting erupted Wednesday at the strategic oil town of Brega, an AFP correspondent said.

Mortar rounds and rockets struck intermittently on both sides of the front at Brega, one of the three main focuses in the Libyan conflict, with the other two at the enclave of Misrata to the east and the Nafusa mountains to the west.

Brega is home mainly to oil facilities, refineries, ports and a residential area.

The United States and EU ratcheted up the pressure on Gaddafi, with Washington lobbying African states to give up support of the Libyan strongman, while the EU imposed new economic sanctions.

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.

In Brussels, the European Union slapped new sanctions on the Gaddafi regime, targeting two "economic entities" linked to human rights abuse, EU foreign policy chief Catherine Ashton said.

An asset freeze and visa ban were extended to "two further entities closely linked to the perpetrators of the serious human rights abuses in Libya, given the gravity of the situation," Ashton said in a statement.

She gave no further details but a French official said on Tuesday the restrictions would target Al-Sharara, which operates in the oil sector, and an administrative organisation linked to Gaddafi.

Six port authorities, 49 entities and 39 people are now subject to a freeze of their funds and financial resources in the EU, Ashton said.

In addition, the same 39 persons, which include Gaddafi and several family members, are banned from entering the EU.

With Britain being one of the key powers engaged in efforts to get Gaddafi to step down, the Libya regime lashed out at Prime Minister David Cameron, calling him to resign over the riots that have hit England over the past few days.

"Cameron and his government must leave after the popular uprising against them and the violent repression of peaceful demonstrations by police," Libya's JANA quoted Deputy Foreign Minister Khaled Kaaim as saying.

He called on the "UN Security Council and the international community not to stay with its arms crossed in the face of the flagrant violation of the rights of the British people."

Nightly riots which began in London and quickly spread to other major cities have gripped Britain since Saturday when an angry crowd marched to demand justice after a 29-year-old man was shot dead by police.

On Tuesday, the Libyan regime also 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.

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

NATO insisted the raids were "legitimate" and said it had no evidence of civilian deaths.