Forces loyal to Moamer Kadhafi launched a fierce counter-attack in the fugitive ex-Libyan leader's stronghold of Bani Walid on Saturday while putting up fierce resistance in his hometown of Sirte.
Despite the heavy fighting, Ahmed Bani, the interim government's military spokesman, said it was only a "matter of days" before the two towns are captured.
Kadhafi forces fired rockets and scored casualties when they shelled a position of government fighters several kilometres (miles) from the centre of the oasis town of Bani Walid, 180 kilometres (110 miles) southeast of the capital.
"They swept the position after targeting us with Grad rockets," said National Transitional Council fighter Omar Ali Ramadan, who said he and others had managed to take cover at an abandoned house.
"At least seven rockets hit our position."
An AFP reporter saw at least three wounded people being carried into a field hospital, one of whom subsequently died amid cries of "Allahu Akbar," (God is greatest).
In Bani Walid, another AFP reporter quoted medical and NTC sources as saying six NTC fighters were killed on Friday and 20 wounded.
The NTC said its fighters had entered Bani Walid, but they made a "tactical withdrawal" on Friday evening due to sniper fire.
"It is useless to hold on to positions overnight in a hostile environment," a commander had told AFP.
In Sirte, NTC forces swept further into the city before retreating under heavy artillery fire after two hours of clashes. At least 6,000 fighters battled in and around one of the ousted Kadhafi's final strongholds.
"The situation at the roundabout is pitiful. There is no central command, we are retreating to regroup and re-enter again from three fronts," said Al-Dhahira Brigade commander Saleb Abu Shaala.
He said the clashes erupted at about 10:30 am (0830 GMT) and that Kadhafi's forces used heavy artillery and rockets against them.
Doctors at a field hospital reported at least 10 killed and 40 wounded in the fighting in Sirte.
Also, French freelance video journalist Olvier Sarbil was seriously wounded when he was struck by shrapnel, said an AFP reporter, who had no further details of the incident.
Commander Salem Jeha, a member of Misrata Military Council, told AFP that "we are now concentrated in a handful of buildings in the city and on the outskirts including Wadi Abu Hadi, where Kadhafi's forces are concentrated."
He said NTC combatants seized Sirte airport late on Friday, a claim disputed by one fighter, and added that there was "no possibility for them (Kadhafi's forces) to continue their resistance."
Abdel Nasser al-Sheikh, also of the Misrata Military Council, charged that Kadhafi troops "are firing from the Bin Hamal mosque. We cannot attack this place."
"There is progress but snipers remain a problem," said fighter Fatha Allah.
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"We reached up to the city centre but there are families there."
Front-line fighters and commanders gave contrasting reports of progress in Sirte, with men on the ground acknowledging they were facing a tough enemy and those in charge downplaying the pockets of resistance.
"We don't even have five percent of Sirte because we just go in and out," said fighter Abdul Rauf al-Mansuri.
He said that despite the massive deployment of armed vehicles in and around the town only a minority of the men were doing the heaviest of the fighting.
"Only ten cars with men go in at a time while hundreds hang back away from the front line shouting God is greatest," he said.
Mansuri added that after night-time clashes NTC forces did not control the airport and that they had also lost their advantage by pulling back at night, giving Kadhafi's men time to rearm.
While the former rebels gained major political victories on Friday with an easing of UN sanctions and receiving Libya's UN seat, they have yet to dominate the battlefield completely.
However, the NTC's Bani claimed that "in a few days the situation will completely change in Sirte and Bani Walid which will be under our control."
Speaking at a press conference in Tripoli, Bani said the "geographical nature and the strong presence of snipers" in Bani Walid prevented a quick conquer of the oasis city, 180 kilometres (110 miles) southeast of Tripoli.
"We managed to enter the town on the north side that we control. We have advanced towards the centre but we were attacked by snipers and mercenaries who have launched rockets from the mountains," he said.
In Sirte, the other bastion located 370 kilometres (230 miles) east of Tripoli, Bani confirmed that "revolutionaries took control of the airport and a major air base.
"Since these two well-guarded positions were taken, the rest of the city would be easy to conquer," he added.
For the remaining bastion of Sabha, 750 kilometres (470 miles) south of Tripoli, Bani said that "when the mercenaries hear that the regime fell in Sirte and Bani Walid, they will act differently," suggesting their possible surrender.
Bani predicted that "in the coming days all of Libya will be entirely under the control of the revolutionaries."
Western nations that were at the forefront of the push for sanctions and help for the rebels hailed the "historic" double breakthrough at the UN for the NTC.
The Security Council unanimously passed a resolution to ease an assets freeze and arms embargo against the new Libyan government, as well as the national oil company, central bank and sovereign wealth fund.
But Resolution 2009 maintained sanctions against Kadhafi and a no-fly zone which has been used to justify NATO air strikes against his forces.
It also established the UN Support Mission in Libya (UNSMIL) to help the interim government arrange elections and write a new constitution.
In February, just after the outbreak of the revolt against Kadhafi, and again in March, the council passed resolutions allowing for sanctions and measures to protect civilians.
The UN General Assembly voted to let the interim government take up Libya's UN seat. That will allow NTC leader Mustafa Abdel Jalil to attend next week's UN gathering in New York, where he will meet US President Barack Obama and other world leaders.