US officials have have foiled an attempt on the life of President Donald Trump by intercepting an envelope addressed to the White House that contained the poison ricin.

The letter, believed to have come from Canada, was spotted at a government facility in Washington that screens all mail addressed to the White House.

A preliminary investigation indicated it tested positive for ricin, a poison extracted from castor beans that has been used in other assassination attempts and terrorism incidents, the official says.

Federal investigators launched an immediate investigation to determine where the enveloped originated  and who posted it. The FBI, the Secret Service and the US Postal Inspection Service were leading the probe.

In a statement, the FBI said agents were working to investigate “a suspicious letter received at a US government mail facility” and that there is “no known threat to public safety”.

There have been numerous incidents involving envelopes mailed with ricin to US officials.

In 2018, a Utah man, William Clyde Allen III, was indicted for making ricin-related threats, including mailing a threat against Trump and other federal officials including FBI Director Christopher Wray. Allen remains in custody.

In 2014, a Mississippi man was sentenced to 25 years in prison after sending letters dusted with ricin to president Barack Obama and other officials.

Also in 2014, a Texas actor was sentenced to 18 years jail for mailing letters containing ricin to Obama and former New York mayor Michael Bloomberg.

-with AAP