THE Pentagon says it has received mail suspected of containing the deadly poison ricin, including an envelope that an official said was addressed to United States Defence Secretary Jim Mattis.

The Pentagon said in a statement it had put its mail facility under quarantine and that the FBI was analysing the envelopes.

In a statement on Tuesday evening, the Secret Service confirmed that a third suspicious envelope had been sent to the president on Monday, but was not received at the White House, nor did it ever enter the White House.

The agency did not disclose any details about what was in the envelope or where it was received. The White House had no comment.

US officials, speaking on condition of anonymity, said the envelopes initially tested positive for ricin after being detected by Pentagon police on Monday at a mail sorting facility.

The other letter was addressed to the US Navy’s top officer, Admiral John Richardson, one official said.

Neither envelope entered the Pentagon. The screening facility is on the Pentagon compound but not in the main building on the outskirts of Washington DC.

Ricin is found naturally in castor beans but it takes a deliberate act to convert it into a biological weapon.

Pentagon spokesman Col. Rob Manning said all US Postal Service mail received at the screening facility on Monday was under quarantine and “poses no threat to Pentagon personnel.”

Ricin can cause death within 36 to 72 hours from exposure to an amount as small as a pinhead.

It is 6000 times more potent than cyanide, with no known antidote.

US government buildings have sporadically received packages with suspected ricin, including in 2013, when ricin-laced letters were addressed to a US senator, the White House and a Mississippi justice official.

Shortly after the September 11, 2001 attacks in New York and Washington, letters containing another deadly substance - anthrax spores - were mailed to the Washington offices of two senators and to media outlets in New York and Florida.