|Opposition fighters have also taken control of an oil refinery in the port city of Az Zawiyah [Reuters]
Libyan rebels have reportedly taken control of the centre of the oil-rich eastern town of Brega, located east of Tripoli.
"It was in the early hours of Saturday morning that opposition forces were able to take the centre of Brega," Al Jazeera's Jacky Rowland reported from Benghazi.
"They were holed up in residential districts outside the town for a couple days. Very early on Saturday morning they moved in."
Questions remained about their control of the oil facilities there, she said.
"We are getting conflicted reports, though, about whether they have been able to get control of the oil and gas facility in Brega," she said.
"Initial reports from military commanders both here in Benghazi, and the frontlines, said yes, the rebels had taken that facility.
"But we are also hearing conflicted rumours that in fact a couple of vehicles went out, found the oil depot apparently deserted, and became suspicious, fearing possible booby traps and ambushes."
Clashes at Tunisia-Libyan border
Meanwhile on Saturday, Tunisian security forces said armed forces fought through the night against a group of armed Libyans who had infiltrated into Tunisia.
One security source said the group were driving vehicles with weapons and had been intercepted in the desert on Friday night. The fighting, which was continuing on Saturday, had caused several casualties.
A military source confirmed that the armed men were Libyans. He did not say whether they were supporters of Libyan leader Muammar Gaddafi or rebels fighting to topple him.
The military source said a Tunisian military helicopter had crashed on Saturday morning in the border area, killing the pilot and co-pilot. He did not say whether the crash, blamed on a mechanical fault, was linked to fighting with the infiltrators.
The Tunisian army has reinforced its presence and intensified its operations in the border area in recent days.
Libya's rebels have advanced this week, cutting off Gaddafi's forces near the Tunisian border from their supply lines to Gaddafi's capital Tripoli.
The latest advance by the rebel forces come as Libyan opposition wrested control of the strategic cities of Az Zawiyah, Zlitan and as they pushed closer to the stronghold of Gaddafi.
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"Az Zawiyah is free," rebels said on Friday as they took up positions in its hospital hours after securing the centre of the town.
Fighting continued late on Friday as Gaddafi forces launched a fierce counterattack along the coastal highway 50km west of Tripoli.
Sustained blasts from rocket-propelled grenades, mortars and anti-aircraft guns were heard from the direction of city's central square as a black column of smoke rose into the evening sky, the Reuters news agency said.
Reuters said that opposition fighters in city's central square exchanged heavy fire with Gaddafi forces occupying a floor of the city's main hospital nearby before driving them out.
This comes a day after rebels took complete control of Az Zawiyah's key oil refinery.
To the east, rebels fought bloody street battles in the city of Zlitan, suffering heavy casualties, Reuters said.
The assault on Zlitan, roughly 150km east of the capital, began around 7:30am local time [0530GMT], and "at 1:00pm local time our information indicates that the rebel troops entered the city centre", the information centre for Misrata military council said in a statement on Friday.
At least 26 rebels are reported to have been killed in the fighting for Zlitan, as forces loyal to Gaddafi used tanks and heavy weapons to repel the attack. Another 150 opposition fighters were reported injured.
The rebels said between 40 and 50 of Gaddafi's forces were also killed in the fighting.
Government troops have been fighting rebels in and around Zlitan for months. The town is a major obstacle in the path from the nearby city of Misrata trying to make their way to Tripoli.
Al Jazeera's Andrew Simmons, who visited Zlitan, confirmed the rebel victory there and said there were scenes of jubilation.
"The rebel fighters took heavy losses, they came under fire from artillery and rockets but they moved forward," Simmons said.
"After fighters from Misrata moved in, opposition fighters within Zlitan rose and took on, in small groupings, the Gaddafi forces.
"The Gaddafi troops pulled out leaving ammunition and a lot of equipment behind."
Foreigners to be evacuated
As fighting intensified, the International Organisation for Migration announced plans to start evacuating "large numbers'' of Egyptians and other foreigners, including some journalists, from Tripoli in coming days.
|The capture of Zlitan, 150km east of the capital Tripoli, is a major boost for rebel fighters
NATO issued a statement that said its air strikes had destroyed a command centre, two armed vehicles and five tanks near Zlitan.
Simmons said for more than two months the [rebels] had been stuck on the outskirts of Zlitan.
"It was a major block because there wasn't overall support by the people of Zlitan initially," he said.
"Those civilians who may have been Gaddafi supporters were treated well by the opposition.
"It's a strategic town, [if] they want to advance on to Tripoli. Now they could do it very quickly. They have a clear run on this coastal road of almost 60km."
The rebels claimed on Thursday they had captured the 120,000-barrel-per-day refinery in Az Zawiyah, a potential turning point in the six-month war.
Opposition forces also claimed to be in control of the town of Surman, 60km west of Tripoli, and Gharyan, 50km to the south.
Significance of Az Zawiyah
Shashank Joshi, of the Royal United Service Institute in London, told Al Jazeera the rebels have learned from past mistakes to move forward methodically.
"They can't just rush ahead and take ground and then forced to move back," he said.
"They have observed that lesson and I think very effectively, and this is why they are still fighting to clear Az Zawiyah.
"They have taken a number of days to fight their way through to take the refinery and they have worked very hard for that, which is why they are very likely, this time around unlike on previous occasions, to actually hold the ground they had taken.
"The significance of Az Zawiyah cannot be seen in isolation, we have to see it in combination of what’s going on in Gharyan, south of Tripoli and Zlitan to the east.
"And all of these locations can be consolidated and their grip solidified, and we are going to see Tripoli being put in a state of siege."
NATO has stepped up bombings in Tripoli in recent days, while rebels have been seeking to sever supply lines from Tunisia to the west and to Gaddafi's hometown of Sirte in the east in a move to cut off the capital, prompt defections and spark an uprising inside Gaddafi's stronghold.
Source: Al Jazeera and agencies