The United States sees a high probability the Al Qaeda network trying to launch a chemical, biological or nuclear attack within two years, the US government said in a report made public on Monday.

"Al Qaeda will continue to favor spectacular attacks but also may seek softer targets of opportunity such as banks, shopping malls, supermarkets and places of recreation and entertainment," the United States told the United Nations in the report.

"Al Qaeda will continue its efforts to acquire and develop biological, chemical, radiological and nuclear weapons. We judge that there is a high probability that Al Qaeda will attempt an attack using a CBRN weapon within the next two years," said the report.

The report, prepared before last month's triple suicide bombings in Saudi Arabia which killed 35 people, did not say whether it thought such an attack would take place inside the United States or elsewhere.

Dated April 17 but just released by the world body, the report was prepared in response to a UN Security Council resolution requiring the 191 UN member-nations to crack down on Al Qaeda -- by, for example, freezing its assets and tracking its agents -- for its role in Afghanistan leading up to the September 11, 2001, attacks.

The group led by Osama bin Laden is blamed by Washington for the suicide hijack attacks, which killed thousands in New York, Washington and Pennsylvania.

Washington said Al Qaeda remained the top concern of US law enforcement authorities.

There were hundreds of ongoing counter-terrorism investigations in the United States directly associated with the group, most of them on the East and West coasts and in the US southwest, it said.

But the greatest threat to U.. security was possible 'sleeper cells' that have not been identified or detected, it said. "Identifying and neutralising these sleeper cells remains our most serious intelligence and law enforcement challenge," the report said.