At least two packages suspected of containing the deadly poison ricin have been sent to the Pentagon this week.

A spokesman said the suspicious packages were addressed to someone in the Pentagon, the headquarters of the US Department of Defence, in Washington DC.

They were found at a delivery facility on the Pentagon grounds but not inside the main building that includes the offices of the defence secretary.

"As part of the screening process, [authorities] recognised some suspicious packages," the spokesman said.

He stressed that authorities were still waiting for confirmation that the packages contained ricin.


Image: A person holds seeds of the castor oil plant, which contain the deadly poison ricin

In a statement, the FBI said: "Special agents took possession of two suspicious envelopes that had been screened at the Pentagon mail facility. Those envelopes are currently undergoing further testing."

All mail received at the Pentagon's screening facility on Monday has been "placed under quarantine and poses no threat to Pentagon personnel", officials stressed.

Ricin is a poison found naturally in castor beans. If the beans are chewed and swallowed, the released ricin can cause injury.

According to the Centres for Disease Control and Prevention, ricin has the potential to cause death under certain circumstances if it is made into a partially purified material or refined.