U.S. Defense Secretary Robert Gates has defended the legality of U.S. military engagement in Libya, as the 90-day deadline for congressional approval expires.

Gates said in a television interview Sunday that he supports President Barack Obama's assertion that the U.S. is playing a support role in the NATO operation in Libya, rather than directly engaging in a major foreign military operation.

Under U.S. law, the president has 60 days to secure congressional authorization for major foreign military engagements, with an additional 30 days to withdraw if no authorization is granted and no extension of the deadline is approved.

As of Sunday, Congress had neither authorized military action in Libya nor given the president an extended deadline.

Congressional opinion on U.S. involvement in the NATO campaign does not split evenly down party lines.

Republican Senators Lindsey Graham and John McCain both spoke out on U.S. television Sunday in favor of the U.S. military action in Libya. McCain and Democrati John Kerry have introduced in the Senate a resolution expressing support for the Libya campaign.

Conversely, some of the president's Democratic allies have joined Republicans to file a federal lawsuit seeking to end U.S. involvement in the campaign.