Rebels and forces loyal to Libyan leader Moammar Gadhafi are engaged in an intense battle for control of Zawiya, a strategic city located just 50 kilometers west of Tripoli.

Rebels said Sunday that they had advanced into the city's center. But, the Libyan government says Zawiya remains under its control.

Zawiya lies on the main highway between Tripoli and the border with Tunisia. Rebel control of Zawiya would cut off Tripoli from its supply lines.

Opposition fighters backed by NATO airstrikes have been pushing since February to gain control of the capital in their bid to unseat Mr. Gadhafi.

On Saturday, rebels said they had seized the town of Gharyan, less than 100 kilometers south of Tripoli. Later in the day, the Associated Press quoted an opposition fighter who said fighting broke out again after pro-government forces returned with reinforcements.

Also Saturday, a Libyan government official slammed U.N. Secretary-General Ban Ki-moon for comments he made earlier in the week about civilian casualties.

Mr. Ban expressed concern Thursday about the rising number of civilian deaths and urged all sides to “exercise extreme caution.”

Libya's Deputy Foreign Minister Khaled Kaim said that Mr. Ban's remarks were “unacceptable” because they did not single out NATO as being responsible for civilian deaths.

On Tuesday, Libya accused NATO of killing 85 people in a village near the western town of Zlitan. NATO said its warplanes had hit targets in the area the previous day but denied evidence of civilian casualties.