US officials believe that a regional arm of Al-Qaeda is trying to produce the lethal poison ricin in order to use it in future attacks against the United States, The New York Times reported.

Citing unspecified classified intelligence reports, the newspaper said Al-Qaeda's affiliate in Yemen has been making efforts to acquire large quantities of castor beans, which are required to produce ricin.

Intelligence officials say they have collected evidence that Al-Qaeda operatives are trying to move the beans and processing agents to a hideaway in Shabwa Province.

US military commanders have repeatedly expressed concern that the jihadists have been taking advantage of a protracted power vacuum in Sanaa to expand their operations in Yemen.

Four suspected Al-Qaeda members have been killed by army fire outside the southern Yemeni city of Zinjibar, most of which has fallen under the control of Islamist militants, a Yemeni official announced Thursday.

Evidence indicates that Al-Qaeda in Yemen is trying secretly to produce 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 report said.

Ricin is so deadly that just a speck can kill if it is inhaled or reaches the bloodstream.

President Barack Obama and his top national security aides were first briefed on the threat last year and have received periodic updates since then, The Times said.

But senior US counterterrorism officials say there is no indication that a ricin attack is imminent, the paper noted.

These officials also note that ricin's utility as a weapon is limited because the substance loses its potency in dry, sunny conditions which prevail in Yemen, and unlike many nerve agents, it is not easily absorbed through the skin, The Times pointed out.