The machines are intended to prevent terrorists or criminals from smuggling into the US a nuclear bomb or its explosive components hidden in a cargo container.

The monitors now in use can detect the presence of radiation, but they cannot distinguish between threatening and nonthreatening material. Radioactive material can be found naturally in ceramics and kitty litter, but would be of no use in making a bomb, for instance.

The Department of Homeland Security has said the new machines it is developing can distinguish between kitty litter and dangerous radioactive material and produce fewer false alarms than the current ones.

The new ones are also better at detecting lightly shielded material. But the machines perform at about the same level when detecting radiological and nuclear materials hidden in a lead box or casing, the most likely way a terrorist would try to sneak the materials into this country, the Government Accountability Office said in a report to be released Monday.

The report by Congress' investigating agency was requested by Senator Joseph Lieberman, the Connecticut independent who is chairman of the Senate Homeland Security and Governmental Affairs Committee.

The report raises the question whether the new machines, at about US$822,000 (S$1.2 million) each, are worth the cost if they're not that much better than current ones that cost about US$308,000.

The department believes the benefits outweigh the additional cost. But department spokesman Sara Kuban said on Friday the department would not comment specifically on a report that was not yet public. She said the new machines still are being tested.

In March, Homeland Security Secretary Janet Napolitano told Mr Lieberman's committee that the administration is not asking for money to purchase these machines in 2010.

The new monitors 'have a limited ability to detect certain nuclear materials at anything more than light shielding levels,' the report said. -- AP