US Secretary of State Hillary Clinton has said that NATO allies were searching for ways to provide funds to Libya's rebels.
"The opposition needs a lot of assistance, on the organisational side, on the humanitarian side, and on the military side," Clinton told reporters on Friday after a meeting of NATO foreign ministers in Berlin.
Clinton said that as well as looking at how to free up assets that could be used by the Libyan rebels, allies were looking at how the rebels could sell oil from sites that were under their control.
The opposition Libyan National Council (LNC), with the help of OPEC member Qatar, has said it has been able to export only a "minimal" amount of crude oil and needs international help to continue overseas shipments.
Rebel efforts to secure supplies have been complicated by international sanctions against Libya. Although the rebels have been unofficially excluded from the sanctions regimes, Western firms remain reluctant to do business with the LNC.
Meanwhile, Russia warned NATO not to use excessive military force in Libya and called for a political settlement to the conflict.
Russian Foreign Minister Sergei Lavrov said it was important not to use "excessive military force which will lead to further additional casualties among civilians."
"We believe it is important to urgently transfer things into the political course and proceed with a political and diplomatic settlement," he told a news conference during the NATO foreign ministers' meeting.
Lavrov said there had been cases where the United Nations' mandate on Libya had been exceeded, for example through talk that it could be used to authorise a ground operation, which he said was not the case.
He said Russia wanted to see the warring parties in Libya come to the negotiating table so they could agree on the structure of their country.
However, leaders of Britain, France and the United States have vowed to continue their military campaign in Libya until Muammar Gaddafi leaves power.
In a strongly worded, jointly written article published in newspapers on both sides of the Atlantic, British prime minister David Cameron, French president Nicolas Sarkozy and US president Barack Obama said leaving Gaddafi in power would be an "unconscionable betrayal" of the Libyan people.
"It is unthinkable that someone who has tried to massacre his own people can play a part in their future government," the leaders wrote in an opinion piece released on Thursday.
"So long as Gaddafi is in power, NATO and its coalition partners must maintain their operations so that civilians remain protected and the pressure on the regime builds," they said.
"Then a genuine transition from dictatorship to an inclusive constitutional process can really begin, led by a new generation of leaders. For that transition to succeed, Colonel Gaddafi must go, and go for good."
'Insult to all Libyans'
The reaction from the Gaddafi camp was swift as the Libyan leader's daughter Aisha told a rally in Tripoli, at a family compound bombed by the Americans in 1986, that demanding her father's departure was an insult to the Libyan people.
"Talk about Gaddafi stepping down is an insult to all Libyans because Gaddafi is not in Libya, but in the hearts of all Libyans," she said in a speech on Thursday broadcast live on Libyan television to mark the 25th anniversary of American strikes on the huge complex, which includes military barracks.
In their article, the US, British and French leaders said Misurata was "enduring a medieval siege as Gaddafi tries to strangle its population into submission".
Aid organisations warn of a humanitarian disaster in the city, the only major rebel stronghold in western Libya, where hundreds of civilians are said to have died in a six-week siege.
Rebels have warned of an impending "massacre" by troops loyal to Gaddafi if NATO does not intensify its attacks on government forces in and around Misurata.
Misurata, Libya's third-biggest city, has been the scene of major fighting between rebels and Gaddafi's forces for several weeks.