WASHINGTON -- President Obama has complied with the War Powers Act by conducting a "limited kinetic operation" against Libya, outgoing Defense Secretary Robert Gates said Sunday, defending the administration's decision to forgo authorization from Congress for the military assault.

Gates, who retires at the end of the month, said he was in the National Security Council at the White House when the War Powers Resolution was first passed in 1973, and Obama has acted in a manner consistent "with virtually all of his predecessors."

"I don't think he's breaking any new ground here," he told "Fox News Sunday."

The Obama administration last week raised the ire of many on Congress -- including some who've filed a lawsuit against the administration for violating the constitutional separation of powers -- when it submitted a report to Congress on why it doesn't need its permission.

The report, described by opponents as not passing the "straight-face test," concluded that the U.S. is not involved in "hostilities" because it is "leading from behind" by letting NATO manage the airstrikes. No American troops are on the ground in Libya, though the U.S. provides support with intelligence, reconnaissance and aerial refueling of warplanes.

The Obama administration estimates the U.S. will spend more than $1 billion on the mission by early September.

Asked about whether the administration was or was not involved in "hostilities," Gates said it's all a matter of perspective.

"The way I like to put it is, from our standpoint at the Pentagon, we're involved in a limited kinetic operation. If I'm in (Libyan dictator Muammar al-Qaddafi's) palace, I suspect I think I'm at war."

The international coalition assisting Libyan rebels in their efforts to oust Qaddafi from power is in its fourth month. Before dawn on Sunday, NATO airstrikes began pounding targets in Tripoli as part of a stepped-up campaign.

Under the War Powers Resolution, passed by Congress in 1973, the president must seek lawmakers' approval to continue to engage in hostilities after 60 days and a 30-day extension.

But Gates' siding with Obama puts him at odds with others in the administration who said the resolution did apply to the mission in Libya.

The New York Times reported over the weekend that top lawyers for the department of Defense and Justice told the administration it needed congressional approval for the mission. Obama then accepted a different point of view offered by his own White House counsel and other members of his legal team.

Gates said he would defer to Obama for legal interpretation of "hostilities," but "I'm confident that he would not make judgment along these lines if he were not confident that he was acting in a constitutional manner."

The Associated Press contributed to this report.