The first export of oil from rebel-held areas of eastern Libya for almost three weeks is due to begin later.
Libya's opposition groups are making plans to load a tanker believed to have now docked at a terminal near Tobruk.
It comes as Nato air strikes were reported against pro-Gaddafi forces and rebels gathered near the town of Brega.
Libya's government has remained defiant, with an envoy who is visiting Europe insisting that Col Muammar Gaddafi will not step down.
Meanwhile Saif al-Islam Gaddafi, son of Col Gaddafi, has told the BBC that Foreign Minister Moussa Koussa did not betray Libya by leaving for the UK.
He told the BBC's world affairs editor John Simpson that Mr Koussa had travelled to Britain for health reasons and was being pressured into making allegations about Libya's government in an effort to secure immunity from prosecution.
The Reuters news agency reported that the tanker had arrived at the Marsa el-Hariga export terminal near the town of Tobruk.
There were unconfirmed reports that the tanker en route to Libya was the Liberia-flagged Equator vessel, owned by Greece-based Dynacom Management.
Lloyd's List, the shipping news and data provider, says that some 1 million barrels of oil are expected to be loaded on to the tanker - possibly bound for Qatar.
The small Gulf state has recognised Libya's rebels as the country's legitimate government and has agreed to market oil from rebel areas.
Libya is Africa's third largest oil producer, but exports have dried up since the anti-Gaddafi uprising began some two months ago.
Libya had been exporting 1.6m barrels a day.
Michelle Wiese Bockmann, of Lloyd's List, told the BBC: "The significance is not only that this is the first shipment in 18 days, but it is also a signal that Libya is open to international trade and shipping. It will send a message to other tanker owners."
The high quality crude was worth about $100m (£62m) at current prices, she said.
The oil is then most likely to be marketed to countries such as Italy, which has previously been an important buyer of Libya's output, she added.
Italy's government has also openly backed the rebel administration.
On Monday, European Union officials clarified legal advice on sanctions, saying that oil exports were allowed as long as revenues did not find their way to the Gaddafi government or state oil company.
The collapse of exports from Libya has helped drive up oil prices, which on Monday hit a two-and-a-half year high.
Brent crude rose $2.36 to $121.06, after earlier reaching $121.29. US light, sweet crude rose 53 cents to $108.47, the highest close since September 2008.
On the ground on Tuesday, Nato jets bombed Libyan government vehicles near Brega, reports said, as small groups of rebels assembled on the outskirts of the city.
The air strikes destroyed two in a convoy of eight vehicles, forcing the others back into the centre of Brega, rebels said.
Nato says Western air strikes have destroyed 30% of Col Gaddafi's forces' military capacity.
The ongoing clashes came as Libya's government struck a defiant note.
A senior envoy visiting Europe, Abdelati Obeidi, met Turkish officials on Monday and refused to back down. "Both sides have a rigid stance," a Turkish foreign ministry official told Reuters news agency.
"One side, the opposition, is insisting that Gaddafi should go. The other side is saying Gaddafi should stay. So there is no breakthrough yet."
In Tripoli, Saif al-Islam Gaddafi told the BBC's John Simpson that Moussa Koussa had been allowed to leave Libya, and denied that the former foreign minister knew incriminating details about the Lockerbie bombing or other atrocities.
"The British and the Americans, they know about Lockerbie, they know everything about Lockerbie so there are no secrets anymore," Mr Gaddafi said.
"Come on. The British government say this: you have no immunity unless you co-operate. He [Moussa Koussa] is sick, he is sick and old so if you put it this way, no immunity of course... [he] will come out with the funny stories."
Moussa Ibrahim, a prominent government spokesman, also backed the long-term leader, warning that without Col Gaddafi Libya could slide into civil strife.
"The leader provides Libyan tribes and Libyan population... a unifying figure," he said.
Libya, Mr Ibrahim said, was open to political reform - "elections, referenda, anything" - but "the leader has to lead this forward".