The white, powdery toxin ricin that is "so deadly that just a speck can kill if it is inhaled or reaches the bloodstream," the New York Times said in a report.
Citing classified intelligence reports, it said al Qaeda's affiliate in Yemen has been trying for more than a year to procure large quantities of castor beans, which are required to produce the toxin.
"Intelligence officials say they have collected evidence that Qaeda operatives are trying to move castor beans and processing agents to a hideaway in Shabwa Province, in one of Yemen's rugged tribal areas controlled by insurgents.
"Evidence points to efforts to secretly concoct batches of the poison, pack them around small explosives, and then try to explode them in contained spaces, like a shopping mall, an airport or a subway station," the Times report added.
However, senior officials have said there is "no indication that a ricin attack is imminent," and experts have pointed out that the Qaeda affiliate is still figuring out how ricin can be used as an explosive since it loses its potency in dry, sunny conditions.
President Barack Obama, who was first briefed on the threat last year, has been receiving regular updates on the matter, the report added.
Senior officials said they are "tracking the possibility of a threat very closely," given the Yemeni affiliate's ability to devise plots that have been thwarted earlier including an attempt by the Nigerian underwear bomber.