The opposition is backed by numerous units of the military in the east that joined the uprising, and they hold several bases and Benghazi's airport. But so far, the units do not appear to have melded into a unified fighting force. Gadhafi long kept the military weak, fearing a challenge to his rule, so many units are plagued by shortages of supplies and ammunition.
On Monday, pro-Gadhafi forces retook control of the western border crossings with Tunisia that had fallen under opposition control and they bombed an ammunition depot in the rebel-held east, residents in the area said. The Libyan Defense Ministry denied the bombing.
Regime forces also moved to tighten their ring around two opposition-controlled cities closest to the capital Tripoli — Zawiya and Misrata — where the two sides are locked in standoffs.
An Associated Press reporter saw a large, pro-Gadhafi force massed on the western edge of Zawiya, some 30 miles (50 kilometers) west of Tripoli, with about a dozen armored vehicles along with tanks and jeeps mounted with anti-aircraft guns. An officer said they were from the elite Khamis Brigade, named after one of Gadhafi's sons who commands it. U.S. diplomats have said the brigade is the best-equipped force in Libya.
A resident of Zawiya said by telephone that fighting started in the evening and intensified after sundown when troops loyal to Gadhafi attacked the city from the west and east.
"We were able to repulse the attack. We damaged a tank with an RPG. The mercenaries fled after that," said the resident, who spoke on condition of anonymity for fear of government reprisals.
He said Gadhafi called Zawiya's influential tribal leader Mohammed al-Maktouf and warned him that if the rebels don't leave the city's main square by early Tuesday, they will be hit by warplanes. "We are expecting a major battle," the resident said, added that the rebels killed eight soldiers and mercenaries Monday.
Another resident of Zawiya said he heard gunfire well into the night on the outskirts of town.
In Misrata, Libya's third-largest city 125 miles (200 kilometers) east of Tripoli, pro-Gadhafi troops who control part of an air base on the outskirts tried to advance Monday. But they were repulsed by opposition forces, who included residents with automatic weapons and defected army units allied with them, one of the opposition fighters said.
No casualties were reported and the fighter claimed that his side had captured eight soldiers, including a senior officer.
The opposition controls most of the air base, and the fighter said dozens of anti-Gadhafi gunmen have arrived from farther east in recent days as reinforcements.
Several residents of the eastern city of Ajdabiya said Gadhafi's air force also bombed an ammunition depot nearby held by rebels. One resident, 17-year-old Abdel-Bari Zwei, reported intermittent explosions and a fire, and another, Faraj al-Maghrabi, said the facility was partially damaged. The site contains bombs, missiles and ammunition — key for the undersupplied opposition military forces.
State TV carried a statement by Libya's Defense Ministry denying any attempt to bomb the depot. Ajdabiya is about 450 miles (750 kilometers) east of Tripoli along the Mediterranean coast.
Gadhafi supporters said they were in control of the city of Sabratha, west of Tripoli, which has seemed to go back and forth between the two camps in the past week. Several residents told the AP that protesters set fire to a police station, but then were dispersed. Anti-Gadhafi graffiti — "Down with the enemy of freedom" and "Libya is free, Gadhafi must leave" — were scrawled on some walls, but residents were painting them over.
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