|Rebels claimed to have regained lost ground along the frontlines near Brega in eastern Libya [EPA]
Pro-democracy fighters have regained ground in a new advance on the oil port of Brega in eastern Libya.
Rebels said the loss of ground early this week to forces loyal to Muammar Gaddafi is a normal occurrence in fluid desert wars, and will not prevent them from ousting the Libyan leader.
Meanwhile, NATO stepped up the pace of its air campaign over Libya on Wednesday, a day after facing fierce criticism of not doing enough to protect civilians in Misurata.
The alliance dismissed opposition criticisms, saying the safety of civilians is its top priority and pledged to do everything it can to ensure that.
It accused Gaddafi's troops of hiding tanks, troops and heavy weapons among civilians to stop NATO aircrafts from carrying out air strikes.
But Al Jazeera's Laurence Lee, reporting from Benghazi on Thursday, said that Abdel-Hafidh Ghoga, the vice-chairman of the Libyan National Provisional Council, continues to insist that NATO do more.
"He has certainly strengthened his language, and he even went on to call NATO a 'burden', which is an extraordinary thing to say under the current circumstances."
"There is obviously agreement here among the opposition that if more military gains are to be made, international forces must step up their operations."
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On Tuesday Gaddafi's forces pushed back rebels from Brega in the pro-democracy movement's first significant loss of territory in almost a week.
"This kind of desert fight is very fluid; advancing 20 kilometres and then retreating 20 kilometres is normal in a desert war," Mustafa Gheriani, a rebel spokesman, said.
"Look at the desert war during the second world war, around [the eastern Libyan town of] Tobruk: they were moving by 50 kilometres every day."
Gheriani said "our forces are at the eastern border of the city, the [Gaddafi] militias are inside the city and the fight is going on".
He said Gaddafi's army "has a lot of weapons left" and can threaten Ajdabiya, about 80km further east, "but we hope our resolve and most of all the resolve of NATO will prevent them to do that".
Hillary Clinton, the US secretary of state, rebuffed a personal appeal from Gaddafi to Barack Obama, the US president, saying the Libyan leader should impose a ceasefire, withdraw his forces and go into exile.
The White House confirmed Gaddafi had written a letter to Obama but did not disclose its contents. The Associated Press news agency, which first reported the letter, said Gaddafi had appealed to Obama for a ceasefire in a rambling, three-page letter.
"Mr Gaddafi knows what he must do. There needs to be a cease-fire, his forces need to withdraw from the cities that they have forcibly taken at great violence and human cost," Clinton said at a news conference with Franco Frattini, the Italian foreign minister, on Wednesday.
"There needs to be a decision made about his departure from power and ... his departure from Libya.
"I don't think there is any mystery about what is expected from Mr Gaddafi at this time. The sooner that occurs, and the bloodshed ends, the better it will be for everyone."
Targeting oil fields
Attacks by government troops this week have also halted production in rebel-held oil fields, just as a tanker with the first shipment of crude left Tobruk on Wednesday.
The rebels have a deal to export oil via the Gulf state of Qatar and use the profits to pay salaries and buy food, medicine and arms to fight Gaddafi.
Ghoga, the rebel official, said groups of armoured vehicles attacked the oil field of Messla and of Sarir earlier this week.
"I think we will not depend on oil revenues in the coming stage because our production has been affected in this crisis."
He said that while the extent of damage remains unclear, the rebels can no longer sustain the 100,000 barrels a day they had been producing. By contrast, in 2009, Libya produced 1.65 million barrels of oil per day.
The rebels still have about one million barrels in storage in Tobruk, which is being exported through the Qatari deal.
"Colonel [Gaddafi] seeks to deprive us of even this by hitting the oil fields that feed this port. This is our wealth and we have to protect it," Ghoga said.
The two fields are part of the massive Sirte Basin region, which is one of the world's largest oil fields and holds 80 per cent of Libya's oil reserves.
Sarir field was discovered in 1961 and is the largest oil field in the country, with estimated reserves of 12 billion barrels. A pipeline carries its oil north to Tobruk.
Messla, discovered in 1971 and just 40km north of Sarir, is estimated to hold three billion barrels of oil.
Libya has the largest proven oil reserves in Africa, even more than Nigeria, at an estimated 46.4 billion barrels as of January 2011, according to Oil and Gas Journal.
Source: Al Jazeera and agencies