Santiago, Chile (CNN) -- President Barack Obama repeated Monday that Moammar Gadhafi "needs to go," but he acknowledged the Libyan dictator may remain in power for some time because the allied military mission in North Africa has a more narrow mandate of just protecting innocent civilians.
"Our military action is in support of an international mandate from the Security Council that specifically focuses on the humanitarian threat posed by Colonel Gadhafi's people," Obama said at a news conference here.
Obama alluded to the fact that U.N. Resolution 1973 passed on Thursday restricts the U.S. and its allies from seeking regime change and directly ousting Gadhafi from power.
But, he noted, "Now, I also have stated that it is U.S. policy that Gadhafi needs to go."
But Obama said he's still hopeful that other "tools" the administration has used, such as freezing billions in Libyan assets, will eventually help the Libyan people push Gadhafi out.
Obama's comments show the delicate balancing act facing the administration as he tries to adhere to the tight U.N. mandate while knowing the mission is unlikely to be seen as a true success around the world unless Gadhafi goes.
"There are a whole range of policies that we are putting in place that have created one of the most powerful international consensuses around the isolation of Mr. Gadhafi, and we will continue to pursue those," Obama said. "But when it comes to the military action, we are doing so in support of U.N. Resolution 1973 that specifically talks about humanitarian efforts, and we are going to make sure we stick to that mandate."
Obama also defended his decision to launch U.S. military action during his travels here in Latin America as necessary because of the urgency of the humanitarian mission in Libya. He gave a specific timetable for the U.S. pulling back from a lead role in the fighting and handing off management of the no-fly zone to NATO allies.
"We anticipate this transition to take place in a matter of days and not in a matter of weeks," Obama said in a comment that echoes what top White House aides have vowed since the mission began Saturday.
Obama said precisely how many days will be determined by military commanders once the first major phase of the mission is completed. He defined the goal as wiping out Gadhafi's air defenses in order to put the no-fly zone into place followed by the U.S. and its allies making sure the "humanitarian aspects of the mission can be met."
But top Republicans -- and some of the president's fellow Democrats -- continue to question whether the mission really is clearly defined and justified.
"I do not understand the mission because as far as I can tell in the United States there is no mission and there are no guidelines for success," Sen. Richard Lugar, R-Indiana, said on "John King USA" in an interview taped for Monday's evening's program. "That may well be true with our allies although conceivably they may have other missions in mind and simply try to get Security Council approval to proceed."
A veteran Democratic congressman normally friendly to Obama's agenda also blasted the decision to commit U.S. forces to the conflict in Libya during an appearance on Monday's "American Morning" on CNN.
"This is a civil war," said Rep. Stephen Lynch, D-Massachusetts. "For the United States to commit its sons and daughters in military action, especially while we have two wars going on already, normally it would require some direct and imminent threat against the United States. That's just not in evidence right now."
Part of the frustration on Capitol Hill stems from a feeling among some lawmakers that they were not consulted enough before the operation commenced, even though Obama has stressed that he informed key congressional leaders in a private meeting at the White House on Friday. A top White House aide also phoned congressional leaders on Saturday, as the mission officially began.
On Monday, the White House also went through the formality of sending a two-page letter from Obama to congressional leaders officially informing them of the military operation.
In the letter, Obama stressed that he believes he had the authority to launch the mission because of the U.N. mandate, and he noted that he has not committed any U.S. ground troops to the fight.
"I am providing this report as part of my efforts to keep the Congress fully informed, consistent with the War Powers Resolution," wrote Obama.
He added that he believes the mission is in the "national security and foreign policy interests of the United States pursuant to my constitutional authority to conduct U.S. foreign relations and as commander in chief and chief executive."
But lawmakers in both parties are also pressing for details on the endgame, especially given the ambiguity about how long Gadhafi will remain in power.
Obama's comments on Gadhafi were similar to what the commander of the U.S. Africa Command told reporters earlier Monday at the Pentagon, when Gen. Carter F. Ham said he "could see accomplishing the military mission which has been assigned to me," even though Gadhafi remains in power.
"Is that ideal? I don't think anyone would say that that is ideal, but I could envision that as a possible situation -- at least for the current mission that I have," Ham said.
Obama was speaking at a joint news conference here with Chilean President Sebastian Pinera on the third day of his five-day tour of Latin America. After starting Saturday in Brazil, Obama heads Tuesday to El Salvador before returning to Washington on Wednesday.
Obama said he had no regrets about authorizing U.S. military force in Libya, saying it was a fast-moving situation and he and U.S. allies had to act quickly to deal with a humanitarian crisis.
"With respect to initiating this action while I was abroad, keep in mind that we were working on a very short time frame, and we had done all the work, and it was just a matter of seeing how Gadhafi would react to the warning that I issued on Friday," said Obama, who departed Washington late on Friday night and arrived in Brazil early Saturday.
"(Gadhafi), despite words to the contrary, was continuing to act aggressively against civilians," Obama said. "After consultation with our allies, we decided to move forward."
Obama added that while he flew to Brazil, "the plan that had been developed in great detail, extensively, prior to my departure, was put into place" by Defense Secretary Robert Gates and Joints Chief Chairman Adm. Mike Mullen.
Earlier on Monday Obama had a one-hour secure conference call with his national security team aboard Air Force One as he flew here from Rio de Janeiro.
National Security Adviser Tom Donilon and White House Chief of Staff William Daley joined Obama aboard Air Force One for the briefing.
Ham joined the meeting by secure communications, as did Secretary of State Hillary Clinton, Gates and Mullen.