The U.S. government said on Thursday all packages brought to airline counters for shipping must be screened for explosives before being loaded on passenger planes under new anti-terrorism rules.

"This is yet another way in which we are continuing to constantly raise the level of security for aviation," Homeland Security Secretary Michael Chertoff told a news conference at Boston's Logan International Airport, from which two of the hijacked planes used in the September 11 attacks took off.

The security directive, which is effective immediately, subjects all express cargo -- envelopes and packages checked at airline ticket and cargo counters -- to the same explosive detection screening used for checked baggage, Chertoff said.

Roughly 15 percent of all air cargo is transported in the bellies of passenger planes. Cargo brought to airline counters by shipping companies or individuals for express delivery accounts for a small percentage of the total, Chertoff said.

Most carriers already voluntarily send this material through the explosive detection machines used for checked bags. The security directive covers those that do not follow this practice and standardizes procedures.

Chertoff said the legal requirement will apply to all airlines at all airports across the country.

Security officials say counter-to-counter cargo is particularly vulnerable because the sender can be reasonably certain which flight will transport the package.

Other cargo is vetted through the government's "known shipper" program in which shippers must meet specific security protocols for sending goods.

Some cargo bound for passenger jetliners is also checked for explosives by bomb-sniffing dogs or physically inspected by Transport Security Administration officials, who have expanded security checks beyond Boston's airport to key bus and rail hubs under a plan also announced on Thursday.