WASHINGTON -- A Homeland Security official says voluntary security measures at chemical plants don't go far enough to protect them from terrorist attacks.

Robert Stephan, the agency's acting undersecretary, planned to ask lawmakers Wednesday to clarify the government's regulatory authority over security of the chemical industry.

The industry has resisted such regulation, saying voluntary protections have helped secure the nation's 15,000 privately operated chemical plants from terrorism.

In written congressional testimony obtained Tuesday by The Associated Press, Stephan says, ''While most companies have been eager to cooperate ... it has become clear that the entirely voluntary efforts of these companies alone will not sufficiently address security for the entire sector.''

''We now have greater clarity about the tasks ahead, tested tools and a more considerable knowledge-base that will help close potential security gaps,'' Stephan says in remarks prepared for a hearing Wednesday of the Senate Homeland Security and Governmental Affairs Committee.

Counterterror experts put the chemical industry at the top of the list of likely terror targets. But congressional investigators have revealed spotty results in how well the chemical industry is prepared to respond in the event of an attack.

About one-fifth of the nation's chemical facilities are close to population centers. Homeland Security has identified 297 chemical facilities where a toxic release could affect 50,000 or more people.