The U.S. is preparing for an expanded campaign against al Qaeda in Yemen, mobilizing military and intelligence resources to enable Yemeni and American strikes and drawing up a longer-term proposal to establish Yemeni bases in remote areas where militants operate.

The developments are part of a U.S. scramble to step up the hunt for members of al Qaeda in the Arabian Peninsula, the terrorist organization behind a recent failed attempt to blow up two planes over the U.S. using bombs hidden in cargo.

Limited...