As NATO defense ministers gathered Wednesday to consider options for action against Libyan leader Moammar Gaddafi, the U.N. special rapporteur for torture said he is looking into allegations that Gaddafi’s security forces have tortured opponents since Libya erupted in an armed rebellion three weeks ago.

The inquiry, opened by Juan Mendez of the Geneva-based U.N. Human Rights Council, adds to a growing list of international moves designed to pressure Gaddafi, the Bedouin army officer who took power in Libya in 1969 — at age 27 — and since then has run the oil-rich nation with an iron fist and an erratic mix of Arab nationalism, anti-Western activism and declarations of African solidarity.

Mendez told the Associated Press in Geneva that he has demanded explanations from Gaddafi’s government about dissident groups’ allegations that Libyan troops pulled protesters from hospital wards to execute them and fired on peaceful demonstrators as the rebellion gathered steam in mid-February. Since those alleged violations, the U.N. General Assembly has voted to suspend Libya from the 47-nation Human Rights Council.

In a separate probe, the United Nations has asked the International Criminal Court in The Hague to investigate whether Gaddafi has committed war crimes or crimes against humanity in his efforts to put down the rebels, who have gained control of the eastern third of the country and are seeking to move westward to dislodge the 68-year-old leader and his family from power in Tripoli, the capital.

NATO defense ministers assembled Wednesday in Brussels to weigh military options for moving against Gaddafi. Ivo Daalder, the U.S. ambassador to NATO, said the goal was to force Gaddafi to step down, to end clashes between government forces and the rebels, and to promote transition to a representative government in Tripoli.

To push in that direction, he said in a briefing, the 28-member NATO military alliance is planning for a range of possible operations, including delivery of humanitarian aid, enforcing a U.N.-declared arms embargo and imposing a no-fly zone that would prevent Gaddafi’s air force from continuing its bombing raids on rebel positions.

Gaddafi, in an interview broadcast by Turkish television Wednesday, said that Libyans would fight back if a no-fly zone were declared. Although he did not specify the means, specialists have said that Libyan air defenses include about 100 SA-2 and 70 SA-6 radar-guided antiaircraft missiles, in addition to traditional antiaircraft cannons.

A rebel representative in Strasbourg, France, said the breakaway forces want Western nations to declare a no-fly zone but only if it is enforced from bases outside Libya and no foreign ground forces are involved. The official, Mahmoud Jebril, was in Strasbourg seeking support from the European Parliament, some of whose commissions are pushing European governments to recognize the rebels’ National Transitional Council in Benghazi as Libya’s legitimate government.

Jebril and another representative of the rebel council, Ali Essaoui, are also scheduled to meet Thursday with French President Nicolas Sarkozy, whose government has hailed the council’s establishment.

The demand for a no-fly zone was seconded in Dubai by Libyan Prince Muhammad al-Senussi, who claims to be next in line for the throne vacated by the aging King Idris when he was ousted by Gaddafi’s military coup. Senussi, who is not known to have a large following in Libya, also called for airstrikes against Gaddafi’s air force. But he emphasized that sending Western ground troops into Libya would be a mistake, because it would be seen as unwelcome foreign intervention even by many of Gaddafi’s opponents.

NATO’s secretary general, Anders Fogh Rasmussen, said that alliance governments are not considering such intervention. But, he said in an interview with Sky News, “we have asked our military to conduct prudent planning for all possibilities.”

A senior U.S. official in Brussels said the Obama administration has concluded that any kind of military operation — including delivery of humanitarian aid or weaponry to rebel forces — would require a determination that a need exists, that such action has regional support and that it conforms with international law.

The legality of such an operation, he said, would best be established by a U.N. Security Council resolution.

The NATO defense ministers plan to discuss their options Thursday in Brussels, with the possibilities ranging from doing nothing to setting up a no-fly zone. The official, who spoke on the condition of anonymity because he was describing delicate military discussions, declined to give details on the substance of the planning, saying only that the ministers would look at “a full range of options.”

Some specialists have suggested that providing the rebels with portable antiaircraft missiles and armor-piercing rocket-propelled grenades would be an efficient way to help, without the cost and dangers of a no-fly zone. Others see the reported discussions of a NATO no-fly zone as a lever for pressuring Gaddafi, even if it does not materialize.

France and Britain have advocated a U.N. Security Council resolution setting up a no-fly zone, a step that is under discussion at U.N. headquarters in New York. The Obama administration reiterated Wednesday that a no-fly zone would require international support, although not necessarily a U.N. resolution.

“We think it’s very important that there be a U.N. decision on whatever might be done,” Secretary of State Hillary Rodham Clinton said. But a State Department spokesman, asked about Clinton’s remarks, said that although “we’d like to see that U.N. support,” it was “not necessarily” the only vehicle for international backing.

The administration hopes that Russia and China, which have indicated opposition to a no-fly resolution, can be persuaded to agree. But U.S. and European governments say that support from NATO, Europe and the Arab League, which will meet to discuss the issue Saturday, would be sufficient legal justification for a “humanitarian intervention” by NATO or a coalition of willing partners.

Staff writer Karen DeYoung in Washington contributed to this report.