By Caren Bohan and Steve Holland
WASHINGTON (Reuters) - President Barack Obama said on Friday the United States and its allies are slowly "tightening the noose" on Libyan leader Muammar Gaddafi and that a no-fly zone remains an option to put pressure on him.
Obama, accused by critics of reacting too slowly to turmoil in Libya, told a news conference he believes international sanctions, an arms embargo and other measures are having an impact and that all other options remain on the table.
"Across the board we are slowly tightening the noose on Gaddafi. He is more and more isolated internationally," Obama said. "I have not taken any options off the table."
Obama said the world had a duty to prevent a repeat of the atrocities in the Balkans during the 1990s. Developments on the ground in Libya must be watched closely and decisions about potential actions made on a case-by-case basis, he said.
Critics are pushing for more forceful intervention, including direct military aid to outgunned rebel groups made up of enthusiastic but ill-trained civilians and rogue soldiers.
The latest appeal for action came from former President Bill Clinton, who said the United States should enforce a no-fly zone over Libya to allow a fair fight between insurgents and troops loyal to Gaddafi.
"I wouldn't do it if they hadn't asked, but if the (insurgent) leaders are on television pleading for it, I think that we should do it," Clinton told a conference in New York late on Thursday.
U.S. Secretary of State Hillary Clinton, the former president's wife, has emphasized the need for U.N. approval for any intervention in Libya.
Obama said NATO would discuss a no-fly zone next Tuesday but any U.S. military involvement must be weighed carefully.
"Any time I send the United States forces into a potentially hostile situation, there are risks involved and there are consequences and it is my job as president to make sure that we have considered all those risks," Obama said.
Secretary of State Clinton is due to meet next week with representatives of the Libyan opposition. Obama said a U.S. official will be assigned to interact with the Libyan opposition and determine ways the United States can assist it.
He sidestepped a question on whether it would be acceptable to the United States for Gaddafi to stay in power, repeating his view that the longtime Libyan leader should step down.
The top U.S. intelligence official, James Clapper, on Thursday offered a grim assessment of Libyan rebels' chances of ousting Gaddafi, saying the opposition had lost momentum and eventually "the regime will prevail."
If that happens, it would undercut Obama both at home and on the world stage after his public calls for Gaddafi's exit.
Rob Danin, a former State Department official now at the Council on Foreign Relations think tank, said a failure by the United States and its allies to take stronger action could have consequences in enabling Gaddafi to stay in power.
"There is not a very clear cut carrot nor a very clear cut stick here for Gaddafi," Danin said.
He added that Obama's "extraordinary caution" suggested that "he does not want to put the United States at the forefront of dealing with this crisis."
Libyan opposition figures kept up pressure on the White House for further steps.
Ali Aujali, the Libyan ambassador to the United States who broke with Gaddafi in February, urged a no-fly zone, arming Libyan rebel forces and official recognition of a self-proclaimed interim government for Libya.
He said the Libyan National Council, the Benghazi-based opposition group that has declared itself an interim government, was "working on" trying to arrange a meeting with Secretary of State Clinton.
Asked if rebel forces should receive arms despite a U.N. Security Council resolution that bans providing weapons to anyone in Libya, he replied: "This is our right ... it is our right to have weapons to at least stop (Gaddafi's forces)."
The United States announced on Thursday that it was sending civilian disaster relief teams to rebel-held eastern Libya to help with humanitarian efforts.
But that plan could proceed slowly.
The U.S. Agency for International Development, which will take the lead on humanitarian relief efforts, does not yet have personnel inside rebel-controlled areas of Libya, and may not send them for days or even weeks, according to an official with the agency who spoke on condition of anonymity.
The official said members of a USAID team were already deployed to the Egyptian and Tunisian borders with Libya, where they are assessing needs and organizing relief efforts.
(Additional reporting by Arshad Mohammed, Mark Hosenball, Jeff Mason, Patricia Zengerle and Alister Bull; Editing by John O'Callaghan and Eric Beech)