WASHINGTON—The government of Col. Moammar Gadhafi hasn't destroyed significant stockpiles of mustard gas and other chemical-weapons agents, raising fears in Washington about what could happen to them—and whether they may be used—as Libya slides further into chaos.

Tripoli also maintains control of aging Scud B missiles, U.S. officials said, as well as 1,000 metric tons of uranium yellowcake and vast amounts of conventional weapons that Col. Gadhafi has channeled in the past to militants operating in countries like Sudan and Chad.

Current and former U.S. officials said in interviews that Washington's counterproliferation operations against Libya over the past decade have scored gains, in particular the dismantling of Tripoli's nascent nuclear-weapons program and its Scud C missile stockpiles. But the level of instability in Libya, and Col. Gadhafi's history of brutality, continues to make the U.S. focus on the arms and chemical agents that remain, they said.

"When you have a guy who's as irrational as Gadhafi with some serious weapons at his disposal, it's always a concern," said a U.S. official. "But we haven't yet seen him move to use any kind of mustard gas or chemical weapon" during the unrest.

The George W. Bush administration reached a key agreement with Tripoli in 2003 that called for Libya to scrap its weapons of mass destruction programs in return for normalized diplomatic relations. The deal followed the toppling of Saddam Hussein and was viewed as a major victory in the push to rid the Middle East of advanced weapons.