The U.S. has failed to meet the July 1 deadline for scanning all inbound cargo containers for radioactive material, which aims at preventing terrorists from smuggling a nuclear device into the country, a newspaper report said Monday.

Homeland Security Secretary Janet Napolitano informed Congress in May that she was extending a two-year blanket exemption to foreign ports because the screening is proving too costly and cumbersome, the Washington Post reported.

It would cost 16 billion U.S. dollars to implement scanning measures at the 700 or so ports worldwide which ship to the U.S., Napolitano was quoted as saying.

Instead, the Department of Homeland Security (DHS) relies on intelligence-gathering and analysis to identify "high-risk" containers, which are checked before being loaded onto ships, the report said.

Under this system, fewer than half a percent of the roughly 10 million containers arriving at U.S. ports last year were scanned before departure. So far there have been no public reports about any smuggled nuclear material being detected during the checks, except for narcotics and other contraband.

The U.S. Congress passed a law in 2007, demanding all cargo containers that are U.S.-bound be scanned for dangerous materials before they are loaded onto ships before July, 2012.

The failure to meet the deadline has aroused concerns among Congress and experts about the safety of U.S. ports. "I personally do not believe they intend to comply with the law," Rep. Edward J. Markey, co-author of the 2007 law, was quoted as saying. "This is a real terrorist threat, and it has a solution. We can't afford to wait until a catastrophic attack."

Though the DHS says that 99 percent of the containers are scanned for radiation after they arrive at U.S. ports, counterterrorism experts are still worried that the monitors at U. S. ports are not sophisticated enough to detect nuclear devices or highly enriched uranium, which emit low levels of radiation.