Aircraft of the NATO-led alliance also hit targets in Tripoli, where at least six powerful explosions were heard.

It was not immediately clear which targets were hit.

"As long as Gaddafi continues to abuse his people, we will continue and intensify our efforts to stop him from doing so," British Foreign Secretary William Hague said at a news conference in the rebel stronghold of Benghazi in eastern Libya.

Speaking in Benghazi shortly before Hague's arrival, the head of the rebel council Mustafa Abdel Jalil welcomed NATO's deployment of helicopters.

Commenting on the use of attack helicopters, Hague said: "It's part of reinforcing the UN Security Council resolution. We have to deal with them (Gaddafi forces) as effectively as we can and that entails the use of Apache helicopters."

A NATO-led military alliance extended its mission to protect civilians in Libya for a further 90 days this week, and France said it was stepping up military pressure as well as working with those close to Gaddafi to try to persuade him to quit.