A series of powerful explosions rocked Tripoli late Saturday, in what could be new NATO airstrikes over the Libyan capital.

Earlier, NATO announced that it had, for the first time, sent attack helicopters on low-altitude missions against military installations.

British Foreign Secretary William Hague met with leaders of the opposition Transitional National Council in the rebel stronghold Benghazi on Saturday. Hague is one of the highest-ranking officials to visit the rebel-held territory.

The foreign secretary also greeted civilians in one of Benghazi's main squares. Hague is accompanied by British International Development Secretary Andrew Mitchell.

NATO said earlier Saturday that its airstrikes had destroyed about 20 targets, including a radar site and an armed checkpoint. Alliance officials said British Apache and French Tiger and Gazelle helicopters carried out the assaults.

The commander of NATO's forces in Libya, Canadian Lieutenant General Charles Bouchard, said the successful assault demonstrates the “unique capabilities” of the attack helicopters. The use of lower-flying helicopters for attacks could diminish the possibility of civilian casualties, although they also could be vulnerable to strikes by surface-to-air missiles.

Despite this increase in the military campaign, there are new diplomatic efforts to try to persuade Mr. Gadhafi to give up power. French Foreign Minister Alain Juppe said Friday that his country is working with those close to the Libyan leader to get him to leave.

Russia is sending a special presidential representative, Mikhail Margelov, to Benghazi, to meet with the rebels.