The unannounced arrest of an Al Qaeda computer engineer in Pakistan has prompted the terror alert in the United States, a report said on Monday.

Mohammed Naeem Noor Khan, 25, was arrested on July 13, it said, adding he is believed to have used and managed an Al Qaeda communications system where information was transferred via codes.

"Documentary evidence" found after Khan's capture showed in detail that Al Qaeda members had been studying the buildings they wanted to target in New York, Newark and Washington, DC even before the September 11, 2001 attacks, a US official told The New York Times.

A second official was quoted as saying that the information provided a new window into the methods, content and distribution of Al Qaeda communications.

"This for us, is a potential treasure trove," an intelligence expert said on Sunday.

The documentary evidence, whose contents were reported to Washington, DC on Friday afternoon, immediately elevated the significance of other intelligence information gathered in recent weeks that had already been regarded as highly troubling, officials told the paper.

Homeland Security Secretary Tom Ridge on Sunday raised the terrorist alert level for key financial centres, warning that Al Qaeda may attack the International Monetary Fund and World Bank headquarters in Washington and the New York Stock Exchange.