TRIPOLI (Reuters) - 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.
Gaddafi remained defiant, dispatching forces to a western border area amid fears that the most violent Arab revolt may grow bloodier and cause a humanitarian crisis.
Tunisian border guards fired into the air on Tuesday to try to control a crowd of people clamoring to cross the frontier and escape the violence.
About 70,000 people have passed through the Ras Jdir border post in the past two weeks, and many more of the hundreds of thousands of foreign workers in Libya are expected to follow.
Gaddafi's 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."
Secretary of State Hillary Clinton told U.S. lawmakers: "Libya could become a peaceful democracy or it could face protracted civil war."
She said the Obama administration would look into allegations that Gaddafi personally ordered the 1988 Lockerbie bombing, because of new statements by defecting Libyan officials "making it clear that the order came from the very top."
The United States said it was moving ships and planes closer to the oil-producing North African state.
The destroyer USS Barry moved through the Suez Canal on Monday and into the Mediterranean. Two amphibious assault ships, the USS Kearsarge, which can carry 2,000 Marines, and the USS Ponce, are in the Red Sea and are expected to go through the canal early on Wednesday.
The White House said the ships were being redeployed in preparation for possible humanitarian efforts but stressed it "was not taking any options off the table."
"We are looking at a lot of options and contingencies. No decisions have been made on any other actions," Defense Secretary Robert Gates said.
French Foreign Minister Alain Juppe sounded a note of caution, saying military intervention would not happen without a clear United Nations mandate.
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 defense 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 were 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 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.
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. He said there were more than 10,000 volunteers in the city, plus defecting soldiers.
Rebels guarding a munitions store said they feared a direct hit by Gaddafi's warplanes could cause destruction for miles around. But Zwei said pilots appeared to be aiming to miss. "We have complete confidence in the Libyan air force not to hit anything that affects their relatives in the east," he said.
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 abandoned on Monday, and residents of Nalut, about 60 km (35 miles) from the border, said they feared pro-Gaddafi forces were planning to recapture the town.
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."
Across the country, tribal leaders, officials, military officers and army units have defected to the rebels. Sanctions will squeeze his access to funds.
Tripoli is a clear Gaddafi stronghold, but even in the capital, loyalties are divided. Many on the streets on Tuesday expressed loyalty but a man who described himself as a military pilot said: "One hundred percent of Libyans don't like him."
There were queues outside bread shops on Tuesday morning. Some residents said many shops were limiting the number of loaves customers could buy.
The U.N. General Assembly on Tuesday unanimously suspended Libya's membership of the U.N. Human Rights Council because of violence by Libyan forces against protesters.
A U.N. Security Council resolution on Saturday called for a freeze on Gaddafi's assets and a travel ban and refers his 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 surged above $116 a barrel as supply disruptions and the potential for more unrest in the Middle East and North Africa kept investors on edge.
Additional reporting by Yvonne Bell and Chris Helgren in Tripoli, Dina Zayed and Caroline Drees in Cairo, Tom Pfeiffer, Alexander Dziadosz and Mohammed Abbas in Benghazi, Yannis Behrakis and Douglas Hamilton; Christian Lowe and Hamid Ould Ahmed in Algiers, Souhail Karam and Marie-Louise Gumuchian in Rabat and Samia Nakhoul, William Maclean and Alex Lawler in London; writing by Andrew Roche; editing by Angus MacSwan