A letter addressed to US President Barack Obama has been found to contain a lethal toxin, the FBI has revealed, according to Belfast Telegraph.

The US Secret Service said the letter, received at a remote mail screening facility, arrived the same day as a letter intended for a US senator tested positive for ricin.

The FBI says preliminary tests on the letter sent to Obama indicate the presence of the poisonous substance ricin.

The letter is undergoing further testing because preliminary field tests can be unreliable, creating false positives. The letter was intercepted at a facility that is not located near to Obama's White House residence.

The FBI says there is no indication of a connection to the Boston Marathon bombing.