WASHINGTON -- US Secretary of State Hillary Clinton on Wednesday rebuffed a personal appeal from Muammar Gadhafi to President Barack Obama to stop an "unjust war" and demanded the Libyan leader withdraw his forces and go into exile.

Obama received a rambling, three-page letter from Gadhafi asking for a halt to a Western air campaign against his forces, but US officials bluntly dismissed the plea.

"Mister Gadhafi knows what he must do," Clinton told a news conference with Italian Foreign Minister Franco Frattini.

"There needs to be a ceasefire, his forces need to withdraw from the cities that they have forcibly taken at great violence and human cost. There needs to be a decision made about his departure from power and ... his departure from Libya," she said.

A top US official told US lawmakers that Libyan agents could be inside the United States and might try to launch retaliatory attacks.

"We want to make certain that we've identified these individuals to ensure no harm comes from them, knowing they may well have been associated with the Gadhafi regime," FBI Director Robert Mueller said.

Gadhafi's appeal to Obama came as Libyan rebels regained ground in a new advance on an oil port but accused NATO of not doing enough to help them end his 41-year rule.

"We can confirm that there was a letter, but obviously not the first," Obama spokesman Jay Carney told reporters traveling with Obama to Pennsylvania. "The conditions the president laid out were clear, which is action, not words."

He declined to provide details on the letter's contents.

Gadhafi, according to a copy obtained by The Associated Press, is said to have addressed Obama as "our son" and "excellency" and implored him to stop what Gadhafi called an "unjust war against a small people of a developing country."

A US official said Washington had received many letters from Gadhafi over the years and the latest was not being taken any more seriously than the earlier ones.

Obama has sought to avert a humanitarian disaster in Libya and keep turmoil in the North African oil-exporting country from destabilizing the broader region while also trying to limit US involvement. He is mindful of an American public already weary of wars in Iraq and Afghanistan.

Obama has called for Gadhafi to leave but has insisted the United States will not use military force to oust him.

"I don't think there is any mystery about what is expected from Mister Gadhafi at this time," Clinton said. "The sooner that occurs, and the bloodshed ends, the better it will be for everyone."

CLINTON DEFENDS NATO

Clinton defended the performance of NATO forces conducting air strikes to enforce a no-fly zone and protect civilians from attack by Gadhafi's forces in fighting between the longtime North African leader and opposition forces.

"We do know that it is difficult when you have a force such as that employed by Gadhafi that is insinuating itself into cities ... for air power alone to be sufficient to take out those forces," she said. "So given the mission that NATO is performing, it is performing admirably."

The United States took the lead in air strikes when military action was first authorized by the United Nations on March 17 against Gadhafi loyalists attacking civilians. NATO assumed full command of military operations from the United States, Britain and France last Thursday.

NATO's air assault has targeted Gadhafi's military infrastructure but only to protect civilians, not to provide close air support for rebels as part of the military measures authorized by the U.N. Security Council.

Western air power has fashioned a rough military balance in Libya, preventing Gadhafi troops from overrunning the ragtag rebel force dominating the east -- but not forceful enough for the insurgents to advance hundreds of miles (km) to the capital, Tripoli, in the west.

Curt Weldon, a former Republican representative from Pennsylvania with a history of pursuing independent foreign policy initiatives to the consternation of the US government, was in Tripoli to try to persuade Gadhafi to step down.

State Department spokesman Mark Toner said Weldon was on a personal trip and not acting on behalf of the US government.

In an opinion piece in The New York Times, Weldon said he had made the visit at the invitation of Gadhafi's chief of staff and hoped to convince Gadhafi to step down, after which a UN-monitored ceasefire should be implemented.