The United Nations said more than one million people fleeing Libya and inside the country needed humanitarian aid, and conditions in rebel-held Misrata town were particularly worrying following attacks on it by forces loyal to Muammar Gaddafi.
Offering a potential olive branch to rebels seeking to end Gaddafi's long rule, one of his associates appealed to opposition chiefs for dialogue, in a sign the ageing autocrat may be ready to compromise with the unprecedented revolt.
The offer, rapidly dismissed by rebels, coincided with a renewed publicity drive by Gaddafi that warned European nations to the north of the Mediterranean that if he fell "you will have immigration, thousands of people from Libya will invade Europe".
A warplane launched an air strike on the eastern outskirts of the rebel-held oil terminal town of Ras Lanuf 600 km (400 miles) east of the capital Tripoli on Monday, witnesses said.
"There was an aircraft, it fired two rockets there were no deaths," Mokhtar Dobrug, a rebel fighter who witnessed the strike, told Reuters. The attack took place at one of two checkpoints in the city.
The attack fitted the pattern of much of the recent fighting, which has been desultory and erratic, with small groups engaging each other, guerrilla-style, in hit and run raids. Air attacks have been fitful and the bombing often inaccurate.
In some areas, advantage on the ground has swung back and forth without conclusive result.
But the resilience of Gaddafi's troops in the face of protests which started in mid-February and their ability to launch a counter-attack has raised the prospect that the country is heading for prolonged bloodshed.
"It's clear the government feels a sense of momentum on its side," said military analyst Shashank Joshi, an associate fellow at Britain's Royal United Services Institute.
"Government forces have more mobility than the rebels thanks to airlift and a decent amount of road transport.
"That's blunted by the fact that we are seeing extremely poor fighting skills by government forces, and reasonably competent fighting by the rebels."
The United Nations and the European Union are dispatching fact-finding missions to the north African nation, where reports by residents of attacks on civilians by security forces have triggered a war crimes probe and provoked global outrage.
Tens of thousands have fled across the border to Tunisia since the uprising prompted a violent crackdown by security forces.
In Geneva, U.N. aid coordinator Valerie Amos said more than one million people fleeing Libya and inside the country need humanitarian aid.
Amos made clear that her first priority was Misrata, a town of 300,000 which residents said had been attacked at the weekend by government forces with tanks and missiles.
"Humanitarian organisations need urgent access now," said Amos, who was in areas of Tunisia along the Libyan border at the weekend. "People are injured and dying and need help immediately."
The rebels have called for U.N.-backed air strikes against what they say are African soldiers-for-hire used by Gaddafi to crush the uprising against his 41-year-old incumbency.
The government says it is fighting against al Qaeda terrorists and maintains that its security forces have targeted only armed individuals attacking state institutions and depots.
Witnesses said government forces advanced on the rebel-held oil port of Ras Lanuf in a counter-attack that forced residents to flee and rebels to hide their weapons in the desert.
"READY TO DIE"
In Ras Lanuf, one angry man told rebels to go home, arguing that they were bringing fighting closer to oil terminals.
Another complained of the rebels' inexperience, as one opposition fighter lay on his back and fired an automatic weapon at a government warplane flying overhead.
"I believe these youths are ready to die, but they won't make a difference," he said. "Look at the way they're firing at the plane. They have no experience, no leadership and no strategy."
The army was moving down the Mediterranean coastal road east of the recaptured town of Bin Jawad, heading towards Ras Lanuf which is about 60 km (40 miles) away and which has a major oil complex, witnesses told Reuters.
Residents of Ras Lanuf, fearing assault by the army, were leaving in cars laden with belongings on Monday and rebels said they had moved weapons into the desert for safekeeping.
As the rival combatants prepared to resume battle, the authorities launched an appeal to the rebels in the east for dialogue, in the clearest overture yet to their opponents.
Jadallah Azous Al-Talhi, a Libyan prime minister in the 1980s who is originally from eastern Libya, appeared on state television reading an address to elders in Benghazi.
He asked them to "give a chance to national dialogue to resolve this crisis, to help stop the bloodshed, and not give a chance to foreigners to come and capture our country again."
Ahmed Jabreel, an aide to rebel leader Mustafa Abdel Jalil, said: "Any negotiations must be on the basis that Gaddafi will step down. There can be no other compromise."
In an interview with the France 24 television station, Gaddafi said Libya was an important partner for the West in containing al Qaeda and illegal migrants trying to reach Europe,
"There are millions of blacks who could come to the Mediterranean to cross to France and Italy, and Libya plays a role in security in the Mediterranean," he said.
In the West, after what residents said was fierce fighting on Sunday with artillery, rockets and mortar bombs, rebel forces announced they had fought off Gaddafi's forces in the towns of Zawiyah, to the immediate west of Tripoli, and Misrata to the east.
As the conflict escalated in Libya, U.S. crude oil rose to a 2-1/2-year high on Monday.
U.S. crude for April rose as much as $1.90 to $106.32 a barrel, the highest price since September 2008, heightening concerns that high energy prices may derail the global economic recovery. The U.S. government reiterated that it could tap its strategic oil reserves to safeguard economic growth.