Reeling under a sustained land, sea and air assault by Libyan forces, rebels on Friday fled from the strategic oil port of Ras Lanuf, with the leader of opposition national council asking the West to impose a flight and naval blockade on his country claiming that people are being 'cleansed' by the regime.

68-year-old Muammar Gaddafi's forces launched a large-scale offensive in another oil-producing town of Brega as well, unleashing aerial raids.

Rebel forces retreated from Ras Lanuf, although some reports claimed that they retained control of the residential parts of the town. There was a sustained aerial, artillery and mortar bombardment, Al-Jazeera reported.

"There have been a number of casualties. We've seen trucks going along the highway, but being shelled by Gaddafi forces all the way along. I counted...50 shells falling," it quoted a witness as saying, adding that 'it seems like a major offensive'.

While many opposition fighters had now left the town, a 'hard core' was going back to fight, the report said.

Rebels in another eastern city of Benghazi feel the tide 'may be reversing', and that there is a 'realisation that this is going to be a long, long uprising,' it said.

"We are outgunned and are facing cleansing from Gaddafi's forces," Mustafa Abdul Jalil, the head of the rebel council in Libya told BBC as Gaddafi's forces stormed and captured Zawiyah in the West and Ras Lanuf in the East after bloody battles.

Just days ago, rebels were boldly promising to march towards Gaddafi's hometown of Surt and Tripoli. But this week they suffered a series of setbacks and a reversal of fortunes in key towns.

Gaddafi's loyalists were also advancing in a wide arc towards other rebel-held areas in a counter-offensive that has reversed the opposition's advance towards his bastion of Tripoli.

"We asked for a no-fly zone to be imposed from day one. We ask the international community to shoulder their responsibilities. We also want a sea embargo and we urgently need some arms and we also need humanitarian assistance and medicines to be sent to the cities besieged by Gaddafi's troops," Jalil said from his headquarters in the eastern Libya.

Commending France for its decision to recognise his National Council, the Libyan opposition leader asked other nations to follow the lead. "The most effective recognition would be from United States, the UK and Germany, but we also need support of all countries," he said.

The Libyan opposition leaders clamour for help from western countries came as Gaddafi's son Seif al-Islam claimed that victory was in sight and that his forces were poised to uproot the rebels from all areas.

"We're coming," Gaddafi's son declared and referred to people in the eastern part of the country as being held 'hostage by the traitors'.

Al-Jazeera said hundreds of rebels and cars and trucks mounted with machine guns were speeding eastwards on the Mediterranean coastal road in a seemingly disorganised flight from Ras Lanuf.

Quoting medics, it said 400 people have died and 2,000 more wounded in eastern Libya since February 17.

The United Nations has said that more than 250,000 have now fled the fighting since the revolt against Gaddafi started. It says over 1,000 have died in the uprising.

But the rebel chief in his interview to BBC insisted that the anti-regime forces were not disintegrating and remained strong. "Everybody should know that there is no balance between our capabilities and Gaddafi's," he said.

A dismal picture of the rebel fighters was also painted by top American intelligence official James Clapper, who created a stir by saying that Gaddafi's forces would prevail.

Gaddafi's government has warned Tripoli residents and imams of the mosques not to allow any protest after the Friday prayers.

Pro-and anti-government forces were locked in intense fighting for control of several other cities and towns along the coastline to the east of Tripoli, including Bin Jawad.