A debate among members of Congress in the US is hotting up over whether or not to screen all cargo transported in the bellyhold of commercial aircraft as is the case for passenger baggage.

House Democrats are demanding that all air cargo be inspected for explosives and other potentially life threatening items that terrorists could smuggle onboard aircraft within three years at an estimated cost of US$600 per year to the federal government, according to a report by the International Herald Tribune.

This would involve a system of "equipment, technology, procedures and personnel" that worked together to "provide a level of security equivalent" to checked baggage screening.

"Here is a backdoor way into planes," said Massachusetts Democrat and House Representative Edward Markey, who has been the leading voice on conducting 100 per cent inspection of air cargo placed on passenger jets, up from the present level of about 30 per cent.

The Transportation Security Administration is not convinced of the need for such inspections, maintaining that existing security measures for air freight are sufficient to prevent a bomb winding up in the bellyhold.

Transport security officials claim that the high cost of inspecting all air cargo would result in security cuts elsewhere which could lead to lower levels of safety overall.

"If you put all of your resources into making something invulnerable, you end up ironically with less security," said Kip Hawley, the Security Administration's Director.

Those opposed to 100 per cent air cargo inspections warn that it will lead to lower revenues for airlines if they are forced to transport air cargo aboard dedicated cargo flights which may also create delays, slower delivery times and leave retail shelves standing bare. At present, 22 per cent of domestic air cargo is carried on passenger aircraft, or about 2.8 million tons a year, producing $4.7 billion a year in revenue for the airlines.

The existing system revolves around transportation officials' efforts to establish a security zone around cargo consignments from the moment they leave the factory to being put on the plane. The government is of the opinion that they can control air cargo security by keeping a record of who is sending the package and by running background checks on the freight forwarder and air cargo employees who have access to shipments. It is based on the notion that when cargo is airlifted the cargo owner has no idea whether the freight will be carried on board passenger or freighter aircraft.

"It is something that could be easily defeated by somebody with patience," said Jim Hall, the former chairman of the National Transportation Safety Board during the Clinton administration.