|Local residents celebrate after Libyan rebel fighters drove Gaddafi forces from Gharyan, south of Tripoli [Reuters]
Libyan opposition fighters have wrested control of the strategic cities of Az Zawiyah and Zlitan as they pushed closer to the stronghold of Muammar Gaddafi.
"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, Reuters reported.
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 reported.
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 Organization for Migration announced plans to start evacuating "large numbers'' of Egyptians and other foreigners, including some journalists, from Tripoli in coming days.
NATO issued a statement that said its air strikes had destroyed a command centre, two armed vehicles and five tanks near Zlitan.
|Capture of Zlitan, 150km east of capital Tripoli, is a big boost for rebel fighters
Al Jazeera's 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. 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."
Joshi continued: "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 severed Gaddafi's supply route from Tunisia.
Az Zawiyah was one of the first cities to rise up against the Gaddafi regime when the Libyan revolt began in mid-February on the heels of the Tunisian and Egyptian revolutions.
Protests were quickly crushed by the Gaddafi regime, even going as far as razing a local mosque in the main square that rebels used as a meeting point and makeshift hospital.
The Libyan opposition has been seeking to sever Tripoli's 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.
Meanwhile, NATO continued with its air raids in parts of Tripoli. Loud explosions rocked the capital early on Friday, as flames lit up skies near Gaddafi's Bab al-Aziziya compund and army barracks.
Expert analyses Libyan rebels' latest offensive
In Tripoli, a government official said that NATO had killed the brother of Gaddafi's spokesman, Moussa Ibrahim.
The official said Hasan Ibrahim, 25, and others were struck by bullets fired from an Apache helicopter while on foot in Az Zawiyah's central square.
The revolt in Libya began in mid-February, with the rebels quickly wresting control of much of the eastern half of the country, as well as pockets in the west.
The conflict later settled into a stalemate with the rebels failing to budge the front lines in the east since April, and making only minor gains from the areas they controled in the east and in the western Nafusa mountains.
But this week the rebels made enormous gains in capturing many western towns and claiming to control the road from Tripoli to the Tunisian border, the main supply line of the capital.
Joshi, however, said the rebels would need to move carefully.
"I won't expect any kind of rush in next several days and hope they would take all the strategic patience required."