New security rules are being delayed for most cargo shippers because the government must adapt its computer software, the Homeland Security Department said March 4.

The cargo rules require electronic manifests identifying freight shipped by truck, rail, plane and ship to be sent to customs officials before the goods reach the border.

"We recognized right from the very beginning we had to make the system more efficient," said John Considine, director of cargo verification for Customs and Border Protection. "We said we would not implement until we made those changes."

Under the system, electronic information will be compared with law-enforcement and commercial databases to target potentially dangerous shipments. Mislabeled cargo or a shipper's record of past violations might cause cargo to be labeled high risk.

The rules were supposed to go into effect March 4.

Instead, air cargo carriers now have three separate deadlines – in August, October and December – depending on which airports they fly in and out of.

An announcement is expected soon on when the rules for truck and rail cargo take effect, Considine said.

Only a tiny percentage of air, rail and truck cargo is currently inspected.