Jordanian authorities claim to have broken up an Al Qaeda plot to unleash a cloud of chemicals in the heart of the kingdom's capital, Amman, according to CNN.
The attack would have been deadlier than the one in the United States on September 11 2001, the network quoted the Jordanian government as saying.
Among the targets were the US embassy, the Jordanian prime minister's office and the headquarters of Jordanian intelligence, it said.
But the plot was unravelled and the authorities raided an Al Qaeda cell on April 20, in which four people were killed and six arrested. A huge quantity of chemicals was also seized.
On Monday, Jordan state TV broadcast a confession from one of the arrested men, Azmi al-Jayusi.
Jayusi, said he worked for Abu Musa al-Zarqawi, who the US claims is a senior Al Qaeda operative behind some attacks in Iraq.
"I have pledged loyalty to Abu Musa to fully be obedient and listen to him without discussion," Jayousi said in Arabic.
The attack on the intelligence HQ alone could have killed at least 20,000 people, according to reports.
