WASHINGTON - The US is about to pull its attack planes out of the international air campaign in Libya, hoping NATO and others can take up the slack.

The announcement Thursday drew incredulous reactions from some in Congress who wondered aloud why the Obama administration would bow out of a key element of the military strategy even as it was showing results.

"Odd," "troubling" and "unnerving" were among critical comments by senators pressing for an explanation of the announcement by Defense Secretary Robert Gates and Joint Chiefs chairman Mike Mullen.

The American combat missions are to end Saturday. Gates said Britain, France and other NATO countries should be able to handle airstrikes on their own, with the US only in support.