SANTIAGO – The United States Monday sent mixed messages about its objectives in Libya, as President Barack Obama said Moamer Kadhafi must go, but vowed US forces would stick to a narrower UN mandate.

"Our military action is in support of an international mandate from the Security Council that specifically focuses on the humanitarian threat posed by Colonel Kadhafi to his people," Obama told a press conference in Chile.

The missile and air strikes launched at the weekend were "in support of UN Security Resolution 1973, that specifically talks about humanitarian efforts, and we are going to make sure that we stick to that mandate."

But Obama stressed it was "US policy that Kadhafi needs to go" accusing the veteran leader who has ruled the north African country for more than four decades of murdering civilians as he tries to quell an opposition uprising.

The US administration has come under pressure for not spelling out its war aims in Libya, even though Obama -- the nation's commander-in-chief -- has insisted there will be no US boots on the ground.

Observers have fretted that the end-game has not been clearly defined and the US military -- stretched by the conflicts in Iraq and Afghanistan -- does not have the capacity to take on another potentially drawn-out war.

There has also been growing international criticism about the strikes launched by US, British and French forces on Libyan air defense systems as they seek to give the rebels cover and impose a no-fly zone on Kadhafi's forces.

But Obama told lawmakers in a letter: "United States forces are conducting a limited and well-defined mission in support of international efforts to protect civilians and prevent a humanitarian disaster."

And the top US military officer, Admiral Mike Mullen, said in a message on the social network Twitter, the objective of the mission in Libya "remains limited; prevent Kadhafi's ability to harm his own people and enforce a no-fly zone."

UN Security Council resolution 1973 -- the fruit of intense diplomacy to avoid Russian and Chinese vetoes while winning Arab support -- allows for "all necessary means" to support the limited aim of protecting civilians.

Defense Secretary Robert Gates said Libya would be "better off" without Kadhafi in charge, although he added "that is a matter for the Libyans themselves to decide."

"And I think, given the opportunity and the absence of repression, they may well do that," he told the Russian news agency Interfax at the start of a two-day trip to Russia.

"But I think it is a mistake for us to set that as a goal of our military operation," he said, according to a transcript.

The United States is leading the military operations in Libya involving a wide coalition of partners, including the French and the British, but says it is eager to hand over command as soon as possible.

This could happen within days rather than weeks, Obama said Monday, adding he believed NATO would play a role in coordinating the next phase of action, even as the alliance struggles to overcome divisions.

"How quickly this transfer takes place will be determined by the recommendations of our commanding officers that the mission has been completed -- the first phase of the mission has been completed," he said.

"Our initial focus is taking out Libyan air defenses so that a no-fly zone can operate effectively and aircraft and pilots of the coalition are not threatened when they're maintaining the no-fly zone," Obama said.

"The second aspect of this is making sure that the humanitarian aspects of the mission can be met."

Obama also assured top US lawmakers that the military strikes on Kadhafi's "lawless" regime were in the US national interest, amid some criticism that the United States was moving outside of its purview.

"Left unaddressed, the growing instability in Libya could ignite wider instability in the Middle East, with dangerous consequences to the national security interests of the United States," Obama said in a letter to Republican House Speaker John Boehner and Senate President Pro Tempore Daniel Inouye.

He vowed the US strikes on Libyan air defenses and military airfields "will be limited in their nature, duration, and scope."

But the administration left no doubt about the final goal, even if the methods for achieving it remain as yet undefined.

"We're trying to convince Colonel Kadhafi and his regime, and his associates, that they need to step down from power," said State Department spokesman Mark Toner. "That remains our ultimate goal here."