Security-sensitive shipping lines will start using new cargo containers next month equipped with sensors and stronger locks in an effort to thwart tampering by terrorists.

Sensors inside the "smart boxes" can detect only whether the cargo box has been opened, not what is concealed inside, U.S. Customs and Border Protection (search) Commissioner Robert C. Bonner said Thursday.

Shipments destined for U.S. ports will not be required to use the containers, but federal authorities will encourage their adoption to speed inspection checks.

Bonner made the announcement during a seminar for members of the Customs-Trade Partnership Against Terrorism (search), an initiative between business and government.

Members of the partnership, including U.S. importers, foreign manufacturers and freight forwarders, will be allowed faster passage for their shipments through "green lanes" at ports.

Existing containers currently offer little indication of tampering.

"They're dumb as a fence post so we just want to make them smarter," Bonner said.

Capitol Hill is increasingly concerned that the nation's ports are ripe for exploitation by terrorists. Senators Susan Collins, R-Maine, and Joseph Lieberman, D-Conn., sent a letter Wednesday to Undersecretary for Border and Transportation Security (search) Asa Hutchinson raising serious questions about what they perceive as flaws in efforts to identify "high-risk" containers.