Reports of a dirty bomb attack in New York City that sent police into heightened alert at the weekend came from a Jerusalem-based website that claims more than a million daily readers but is criticized by intelligence experts as unreliable.

Giora Shamis and his wife launched Debkafile in 2000 to focus on security, terrorism and military affairs in the Middle East. Shamis claims he and his wife predicted the al-Qaeda attack on the World Trade Center.

Shamis says Debka, which is financed by subscriptions and ad revenue, has a daily readership of 1.3 million. But its reports, published in English and Hebrew, rely on anonymous sourcing and often proved untrue.

"I don't know a single Israeli official who sees the site as a reliable source," said Ronen Bergman, a reporter for Israel's Yediot Ahronot daily who specializes in espionage.

Last week the site reported intercepting al-Qaeda messages on radioactive attacks against US cities, identifying New York, Los Angeles and Miami as potential targets.

"We have about four or five people that are sitting around the clock, 24-hours-a-day, tracking the different al-Qaeda publications," Shamis said. "For us, this kind of story is routine."

But the reaction was not.

Last Friday the New York Police Department increased its radiological monitoring and established checkpoints to examine vehicles around Manhattan.

NYPD Deputy Commissioner Paul Browne said the measures were prompted by a Debka report on online al-Qaeda chatter about a truck packed with radioactive material. By Saturday the department had eased its alert, but Shamis stood behind his report.

Boaz Ganor, an Israeli anti-terror expert, said: "I don't take Debka seriously as a reliable source ... but there are people who take it seriously and that's what makes it dangerous."