"If things go the way they are, I think he could last for more than a month, a few months. That would be disastrous for the rest of Libya," said Mohamed Yousef Al-Megariaf, a critic of the regime who quit his post as Libya's ambassador to India in 1980.

"The reason he is hanging on is not because he is popular, but because of the number of troops who belong to his tribe, the volume of his weapons and his willingness to inflict an unimaginable degree of suffering on the Libyan people," he added.

James Phillips, a senior research fellow at the Heritage Foundation, said he fears that the regime could hold out for years, at least in a defensive manner.

"It appears that the opposition forces are too weak to liberate Tripoli and other strongholds of support for Gadhafi," Mr. Phillips said. "We may be headed into a prolonged civil-war-type situation."

Pro-Gadhafi forces have escalated their offensive in recent days in a seesaw struggle with rebel troops. Over the weekend, residents reported pitched battles in rebel-controlled cities, including Zawiya, Ras Lanuf and Misurata. The regime also conducted airstrikes on Brega and outside Ajdabiya.

On Sunday, rebels in Misurata beat back the fiercest attack so far by Gadhafi forces trying to retake the town. At least 18 people died in the fighting, a doctor told reporters.

However, rebels retreated on Sunday from Bin Jawad on the road to Surt, Col. Gadhafi's hometown and a prize that the rebels are keen to capture.

Brega, Zawiya and Ras Lanuf have key oil terminals, and the provisional rebel government pledged Sunday to honor all oil contracts. Libya's proven oil reserves are estimated at 43.7 billion barrels, the ninth-largest amount in the world, according to Oil and Gas Journal.

Libya's security apparatus is broadly divided into four parts: the regular armed forces, security forces, the police, and revolutionary committees or militias. Since the outbreak of anti-government protests last month, the regime also has relied on Libyan-trained African mercenaries from Chad, Mali, Niger and Algeria to attack unarmed civilians.

The Gadhafi regime has dipped into Libya's significant oil revenue to fund these mercenaries, who initially were sent to fight in other parts of Africa.

The southwestern city of Awbari is the "key entry point" for these mercenaries, said Ibrahim Sahad, a former captain in the Libyan army.

A majority of the military unit in Awbari is composed of soldiers from the nomadic Tuareg tribe from Mali and Niger, Mr. Sahad said.

"This unit is receiving mercenaries who are coming from Niger," he said.

The African Union has condemned the use of force against civilians in Libya and urged dialogue between the groups.

Ironically, in 2009, Col. Gadhafi was appointed chairman of the 53-nation African Union. In 2008, more than 200 African kings and traditional rulers bestowed the title "king of kings" on the Libyan strongman.

Libya's army, navy and air forces are equipped with aging Cold War-era equipment.

"The country's poorly maintained inventories also include some U.S. and Western European arms, including French Mirage fighters and U.S. C-130 transports," according to a Congressional Research Service report.

Libya's military always has been expensive to maintain but never had the training to be broadly effective, said Anthony H. Cordesman, a defense analyst at the Center for Strategic and International Studies.

"It has always had far more equipment than its total manning can actually operate," he said. "You have an army with some 25,000 regulars and some 25,000 conscripts, attempting to man a force with over 2,000 tanks and something on the order of 2,000 other armored vehicles and 2,500 artillery pieces. It frankly is a structure that exists in a dream world."

Col. Gadhafi, who came to power in a coup in 1969, is deeply suspicious of the armed forces and over his four-decade rule has sought to limit its powers.

The security forces control the armed forces' access to ammunition. Mr. Sahad said ammunition is routinely taken away from soldiers when they are not in combat.

The security forces, on the other hand, are well-equipped and devoted to the protection of Col. Gadhafi and his regime.

This force "consists of foreigners or people from Gadhafi's tribe or selected in a specific way to guarantee their loyalty to Gadhafi," said Mr. Sahad.

Libya's best-trained, best-equipped and best-paid force is the elite 32nd Brigade, led by Col. Gadhafi's Russia-trained son Khamis. This brigade is considered key to the regime's survival and has Russian, British and Italian equipment.

"Aside from one elite brigade, the other battalions have always been more a matter of image than reality," said Mr. Cordesman.

The al-Fadeel battalion in Benghazi was responsible for much of the recent bloodshed in the eastern city. Benghazi fell to the rebels after army defectors and armed civilians overpowered the battalion.

Col. Gadhafi and his family are thought to be staying at Bab al-Aziziya, a heavily fortified military barracks on the outskirts of Tripoli.

In 1986, President Reagan ordered an airstrike on Bab al-Aziziya in retaliation for Libya's bombing of a West German disco that killed two U.S. soldiers.

In recent weeks, the Libyan military has been hit by desertions, especially in eastern cities such as Benghazi. How much of the force remains is anybody's guess.

Col. Gadhafi's power is confined to Tripoli and Sirte, where his Gadhadhfa tribe remains deeply loyal.

Meanwhile, aerial bombardments of Brega and Ajdabiya have escalated calls from some Libyans for a no-fly zone over their country.

The Libyan air force has 374 combat aircraft and 18,000 people in the entire force and flies about a third of the hours necessary for minimum proficiency.

"A good part of the force is not operational, which probably explains why a lot of the so-called air attacks have really been by anti-tank helicopters," Mr. Cordesman said.

Mr. Sahad said that with a no-fly zone over Libya, Col. Gadhafi would not be able to use his air force.

"Without his planes, the cities in the east would be able to secure their areas and march toward Tripoli," he said.

But some, including Defense Secretary Robert M. Gates, have cautioned that imposing a no-fly zone would be tantamount to a declaration of war.

Much of the Libyan forces' equipment dates back to the Cold War era, "but you can't ignore it," said Mr. Cordesman.

The Libyan military has 216 major surface-to-air missile-fired units, including about 72 SA-6s and SA-8s, 108 to 144 SA-2s, and some SA-5s and SA-3s.

"Could you freely fly in this kind of environment without suppressing the air defense, which means attacking it? No you couldn't," Mr. Cordesman said.

Gen. James Mattis, the head of U.S. Central Command, told a Senate panel last week that imposing a no-fly zone would be a military operation.

"It wouldn't be just telling people not to fly airplanes," he said.