Forces loyal to the Libyan leader Muammar Gaddafi are moving into rebel territory in the east, capturing an oil installation in the town of Brega.
The manager of the installation said they took control at dawn without using force, but rebel forces later said they had regained control of the town.
Pro-Gaddafi jets also bombed an arms dump in the nearby city of Ajdabiya.
The city's defenders are in a high state of excitement and expecting an attack, a BBC correspondent says.
Meanwhile Libyan TV showed what appeared to be live pictures of Col Gaddafi meeting crowds in a large hall in central Tripoli, as a cheerleader chanted slogans.
In two weeks of unrest the Libyan leader has lost control of large parts of Libya.
The violence has led to a major humanitarian crisis on the Tunisian border, with tens of thousands of foreigners, most of them Egyptian, stranded and unable to get home.
Some 75,000 people have fled to Tunisia since unrest began and 40,000 more are waiting to cross, the UN says.
The United Nations says a mass evacuation is needed and thousands of lives are at stake.
In other developments:
In the capital Tripoli, where Col Gaddafi is still in charge, a fuel tanker overturned causing several large explosions. It is not clear whether they were caused by sabotage
Col Gaddafi's son Saif al-Islam tells France's Le Figaro newspaper "a few hundred people" died in eastern Libya in the early stages of the unrest after police officers "panicked" but denied air strikes against civilians
Two US warships, the USS Kearsarge and USS Ponce, enter the Suez Canal en route for the Mediterranean, after orders from Defence Secretary Robert Gates that they should move closer to Libya
The rebel revolutionary council in Benghazi formally asks the UN to end air strikes by forces loyal to Col Gaddafi, media reports say
Arab League foreign ministers are meeting to discuss a draft resolution rejecting foreign military intervention in Libya. They held a minutes' silence to remember "martyrs" of reform in the Arab world
Rebels determined
Government forces took the oil facility at Brega at dawn on Wednesday without using force.
"It's not an attack. We are OK. The government troops came in to secure the whole area. Our concern is to maintain the facility," Ahmed Jerksi, the manager of the oil installation in Brega, told the Associated Press.
But rebels in Benghazi, the main city in eastern Libya, said they had retaken the town.
"They tried to take Brega this morning, but they failed," spokesman Mustafa Gheriani told Reuters news agency. It is back in the hands of the revolutionaries. He is trying to create all kinds of psychological warfare to keep these cities on edge."
The BBC's John Simpson in Ajdabiya says rebel forces in Ajdabiya have been expecting an attack but with fighting continuing in Brega that may not be imminent.
The rebels are determined to put up a fight but it remains to be seen whether this translates into an organised defence of the city, our correspondent says.
Protesters fear air attacks on the towns they have won and are calling for the creation of a no-fly zone over Libya.
The UK has been investigating the possibility, but the BBC's Barbara Plett at the UN says there is little appetite in the Security Council for such a move.
Meanwhile, UN Secretary General Ban Ki-moon said reports suggested more than 1,000 had died in the unrest so far.
The UN has passed a resolution suspending Libya from its Human Rights Council and accusing it of committing gross and systematic violations of human rights.
Mr Ban said: "These UN actions send a strong and important message - a message of great consequence within the region and beyond - that there is no impunity, that those who commit crimes against humanity will be punished, that fundamental principles of justice and accountability shall prevail."
