U.S. Secretary of Defense Robert Gates said on Sunday that Libya did not pose "an actual or imminent threat" to the United States before the U.S. began its military campaign against the North African country, and regime change there "was never part of the military mission."

"No, no," Gates replied in response to a question about whether he thinks Libya "posed an actual or imminent threat to the United States," adding that "it was not a vital national interest to the United States."X But he told ABC's Jake Tapper on its "This Week" program that it was an interest due to the engagement of the Arabs, the Europeans, the "general humanitarian question that was at stake" and the potential wave of refugees from Libya that "could have destabilized Tunisia and Egypt," where the two countries' long-time leaders stepped down in civilian unrest respectively in January and February.

Secretary of State Hillary Clinton, who appeared with Gates on the program, said "Why -- how could you stand by when, you know, France and the United Kingdom and other Europeans and the Arab League and your Arab partners were saying you've got to do something."

The Obama administration is facing mounting criticism at home both from the media and politicians for not asserting clear goals and consulting intensively before launching military strikes on Libya on March 19 to set up a no-fly zone over Libya as authorized by a UN Security Council resolution.

During his campaign for the presidency in December 2007, Barack Obama told the Boston Globe that "the president does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation."

As to when the military strikes in Libya will end, Gates replied that "I don't think anybody knows the answer to that."

The Americans have been weary of the wars in Afghanistan, which has been dragging on for almost 10 years, as well as in Iraq, which lasted for almost eight years.

Obama has called for Libyan leader Muammar Gaddafi's departure, but regime change is not one of the goals of the military operations.

"Well, first of all, I think you don't want ever to set a set of goals or a mission -- military mission where you can't be confident of accomplishing your objectives," Gates explained.

"And as we've seen in the past, regime change is a very complicated business. It sometimes takes a long time. Sometimes it can happen very fast, but it was never part of the military mission."

U.S. President Barack Obama said on Saturday that the U.S.-led military mission in Libya is "clear and focused" and achieving success.

He told the American public in his weekly radio and internet address that the U.S.-led coalition has taken out Libya's air defenses and prevented Gaddafi's forces from "advancing across Libya."

As part of the administration's publicity campaign, the White House announced that Obama will deliver an address at the National Defense University in Washington on Monday evening to update the American people on the situation in Libya.

In addition, Clinton, Gates, Chairman of the Joint Chiefs of Staff Mike Mullen and Director of National Intelligence James Clapper are scheduled to hold a classified briefing on Libya next Wednesday for lawmakers.