United Nations Secretary-General Ban Ki-moon has expressed concern over the rising number of civilian casualties in Libya's ongoing conflict between rebel fighters and government loyalists.

In a statement Thursday, the secretary-general called on all sides to “exercise extreme caution in their actions, in order to minimize any further loss of civilian life.” Ban also said that “a ceasefire linked to a political process” is the only viable means to achieve peace in Libya.

NATO began launching air strikes against Libyan leader Moammar Gadhafi in March, supporting Libyan rebels who have been fighting against his 42-year autocratic rule since February.

Earlier this week, the head of the U.N. cultural agency UNESCO criticized NATO airstrikes last month against Libyan state television facilities that left three people dead. Director-General Irina Bokova said media outlets should not be targeted in military actions.

NATO officials have defended the bombing, saying the broadcasts were used to incite attacks, and that the U.N. Security Council resolution requires them to use “all necessary” measures to protect civilians.

On Thursday, the French News Agency quoted the NATO commander of Libyan operations as saying Mr. Gadhafi's forces have been weakened and are “no longer able to launch a credible offensive.”

Meanwhile, Libyan rebels said Thursday they captured several residential districts in Brega, a key oil port region.

A rebel spokesman said two of their fighters were killed and at least 16 others wounded in Thursday's fighting for the control of the port city located about 750 kilometers east of Tripoli.

There were also reports that the rebels forces in the west pushed toward the coastal town of Zawiya Thursday — the closest rebels have come to Tripoli, Mr. Gadhafi's main stronghold.