US authorities confirmed an arrest Sunday over an envelope addressed to President Donald Trump that contained a substance identified as the poison ricin.

The suspect was a woman who was carrying a gun when she was arrested by authorities, according to US media reports.

"I can confirm that an arrest was made at the Peace Bridge in Buffalo, New York," Customs and Border Protection supervisory officer Aaron Bowker told AFP, referring to the bridge that connects Canada and the US state of New York.

US media reported Saturday that authorities had intercepted an envelope laced with ricin addressed to Trump.

The letter was discovered earlier last week and did not reach the White House, according to The New York Times and CNN.

Arrest over ricin envelope
Mail addressed to the White House is first inspected and sorted in depots just outside Washington. Photo: AFP / Brendan Smialowski

The Times said it was believed that the letter was mailed from Canada.

Mail addressed to the White House is first inspected and sorted in depots just outside Washington.

CNN said the contents of the envelope were tested repeatedly at one depot and confirmed to contain ricin.

Ricin, which is produced by processing castor beans, is lethal even in minute doses if swallowed, inhaled or injected, causing organ failure.

A government official who was not authorized to speak on the matter told AFP that a court appearance was scheduled for Monday.

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