At least three Libyan opposition fighters have been killed in clashes near the northern town of Zlitan, just 160km from Tripoli, the capital, as government troops fought rebel forces for control of the town.
Several other opposition forces were injured in the fighting on Sunday, Al Jazeera's Andrew Simmons reported, as troops loyal to Libyan leader Muammar Gaddafi continued an assault against anti-government fighters.
Opposition forces were also under attack in the newly captured town of Bir al-Ghanam, a strategic location in western Libya 85km from Tripoli, where Gaddafi forces launched an offensive to regain control of the town.
The opposition forces are attempting to get closer to Tripoli, and they expressed hope earlier this week that they would reach the capital before the end of the Muslim fasting month of Ramadan.
Opposition 'better trained'
Al Jazeera's Zeina Khodr, reporting from Bir al-Ghanam just after rockets began falling on the frontline, said that there was a sense that the opposition fighters were better organised and better trained than earlier in their campaign to topple Gaddafi.
"There is a sense that they know what they lack," she said.
"Volunteers from all over the country are joining the fight. Not just people from the western mountains, rebels from Az Zawiyah and rebels from Tripoli.
"Bir Ghanem is such a strategic town that when they take that town they [will] have access to the main highway that reaches to Az Zawiyah, which is just west of Tripoli."
'Holding the offensive'
Khodr said the greatest challenge for the opposition forces in Bir Ghanem was "holding the offensive". "The most important thing for them now is to reach Az Zawiyah," our correspondent said.
She said that an offensive was not only made in Bir Ghanem by opposition forces on Saturday but that an offensive, which is holding, was also made along the road that led to the city of Surman.
"Rebels advanced some 30km and are only 50km from the city of Surman.
"If they manage to take that town they will be able to cut off Gaddafi's main supply line in the west," she said. "They know that they can get support from inside that city, that rebels there are ready to rise up against the Gaddafi regime but they need help from outside."
Az Zawiyah was the scene of a major uprising by protesters early on in the conflict, which began in February. The protesters took over the city and drove out Gaddafi's supporters, but were then brutally crushed in a long, bloody siege.
However, our correspondent said opposition forces told Al Jazeera that they do not plan to advance further, "especially those at the front leading to Surman. Not until they are able to clear the territory and hold it".
"They know when they reach the big cities it's going to be a whole different fight."
Advance near Zlitan
Meanwhile, near the city of Zlitan, Al Jazeera's Andrew Simmons said the opposition was still in "defensive mode" against Gaddafi's forces.
"Unlike the western mountains, this is staying static. It's a critical position." our correspondent said.
More than 30 vehicles moving at speed had attempted to outflank opposition forces during an intense firefight on Sunday, he said.
But Gaddafi's forces withdrew after realising they were facing the most fierce of the opposition units, the Shaheed unit.
"They withdrew leaving about five vehicles behind. They took a lot of casualties as well apparently but no one knows how many [are] dead."
"Three opposition forces were killed and [they suffered] several casualties. Opposition forces pulled back and we saw a lot of rounds coming in."