Tripoli: Libya could descend into civil war unless Muammar Gaddafi quits, the United States said on Tuesday, its demand for his departure intensifying pressure on the longtime leader after news of Western military preparations.

But Gaddafi remained defiant, dispatching forces to a western border area amid fears that the most violent Arab revolt may grow bloodier and spark a humanitarian crisis.

His son, Saif al-Islam, warned the West against launching any military action to topple Gaddafi, and said the veteran ruler would not step down or go into exile.

"Using force against Libya is not acceptable. There's no reason, but if they want ... we are ready, we are not afraid," he told Sky television, adding: "We live here, we die here."

In Moscow, a Kremlin source suggested Gaddafi, whose authority has unravelled in much of the vast desert country, should step down, calling him a "living political corpse".

In prepared testimony to U.S. lawmakers in Washington, U.S. Secretary of State Hillary Clinton said: "Libya could become a peaceful democracy or it could face protracted civil war."

The U.S. ambassador to the United Nations, Susan Rice, said Washington would apply pressure on Gaddafi until he bows out, working to stabilise oil prices and avert a humanitarian crisis.

She stopped short of saying the Obama administration was ready to impose a no-fly zone over Libya that would prevent Gaddafi using aircraft against rebels fighting against him.

On Monday the United States said it was moving ships and planes closer to the oil-producing North African state. French Foreign Minister Alain Juppe sounded a note of caution, saying foreign military intervention in Libya would not happen without a clear United Nations mandate.

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British Prime Minister David Cameron said it was unacceptable that "Colonel Gaddafi can be murdering his own people using airplanes and helicopter gunships."

General James Mattis, commander of U.S. Central Command, told a Senate hearing that imposing a no-fly zone would be a "challenging" operation that would mean actual attack.

"You would have to remove air defence capability in order to establish a no-fly zone, so no illusions here," he said. "It would be a military operation -- it wouldn't be just telling people not to fly airplanes."

Analysts said Western leaders are in no mood to rush into conflict after the troubled, drawn-out involvements in Afghanistan and Iraq.

"They will be desperate not to place themselves in that situation, unless not doing so would result in even worse massacres," said Shashank Joshi of London's Royal United Services Institute.

Suspicions grew that Gaddafi, a survivor of past coup attempts, did not grasp the scale of the forces against him.

"All my people love me," he told the U.S. ABC network and the BBC on Monday, dismissing the significance of a rebellion that has ended his control over much of oil-rich eastern Libya.

REBELS CLAIM STRENGTH GROWING

Rebel fighters claimed the balance of the conflict was swinging their way. "Our strength is growing and we are getting more weapons. We are attacking checkpoints," said Yousef Shagan, a spokesman in Zawiyah, only 50 km (30 miles) from Tripoli.

A rebel army officer in the eastern city of Ajdabiyah said rebel units were becoming more organised.

"All the military councils of Free Libya are meeting to form a unified military council to plan an attack on Gaddafi security units, militias and mercenaries," Captain Faris Zwei said.

But despite the widespread collapse of Gaddafi's writ, his forces were fighting back in some regions.

A reporter on the Tunisian border saw Libyan troops reassert control at a crossing that was abandoned on Monday, and residents of Nalut, about 60 km (35 miles) from the border, said pro-Gaddafi forces deployed to retake control there.

Mohamed, a resident of rebel-held Misrata, told Reuters by phone: "Symbols of Gaddafi's regime have been swept away from the city. Only a (pro-Gaddafi) battalion remains at the city's air base but they appear to be willing to negotiate safe exit out of the air base. We are not sure if this is genuine or just a trick to attack the city again."

Tripoli is Gaddafi's last stronghold. Tribal leaders, officials, military officers and army units have defected to the rebels. Sanctions will squeeze his access to funds.

BREAD QUEUES

Around the Libyan capital there were queues outside bread shops on Tuesday morning. Some residents said many shops were limiting the number of loaves customers could buy.

"The situation is nervous," said Salah, a doctor. "Of course I am worried. My family is afraid. They are waiting at home. We have been hearing gunfire."

A Tripoli resident said the main state television station, Jamahiriya, was no longer available on its usual satellite channel. He said Libiya, another of Libya's three television stations, which are all state controlled, had told viewers Jamahiriya's signal was being subjected to interference.

It was not clear if the interference was linked to a campaign, led by Libyan exile groups, to persuade satellite television operators to stop carrying Libyan television.

In the opposition bastion of Benghazi, residents said food and other necessities were in good supply.

In Geneva, Russian Foreign Minister Sergei Lavrov called on world powers to fully implement a U.N. Security Council resolution on Libya. The text, adopted on Saturday, includes a freeze on Muammar Gaddafi's assets and travel ban and refers his regime's brutal crackdown to the International Criminal Court.

Libya's National Oil Corporation said output had halved because of the departure of foreign workers.

Brent crude prices pushed back above $114 a barrel as supply disruptions and the potential for more unrest in the Middle East and North Africa kept investors on edge.

At Ras Jdir on the border with Tunisia, Tunisian border guards fired into the air to try to control a crowd of people clamouring to get through a frontier crossing to escape the violence in Libya.

About 70,000 people have passed through the Ras Jdir frontier crossing in the past two weeks, and in the last few days the rate has increased to up to 15,000 per day, said Ayman Gharaibeh, an officer with the U.N. refugee agency.

Revolutions in neighbouring Tunisia and Egypt have helped to ignite resentment of four decades of often bloody political repression under Gaddafi as well as his failure to use Libyan oil wealth to tackle widespread poverty and lack of opportunity.