At least 19 people have been quarantined or examined in hospital after the incidents in Denver, officials said, while all staff in the New Hampshire office were also checked by medical staff.

The incidents come only days before the Democratic Party is to due to hold its annual convention in Denver.

A Federal Bureau of Investigation (FBI) hazardous materials team was dispatched to the scene in Colorado, along with secret service personnel and local authorities, Malcolm Wiley, a US secret service spokesman, said.

However, officials now believe that the Denver letter was a hoax sent by a prison inmate, a US government official told The Associated Press news agency.

'Attack' fears

Barack Obama, the Democratic presidential candidate, and his former rival, Hillary Clinton, will attend the convention in Denver.

A spokeswoman for Obama said they were unaware of any similar incidents at the Democrat's offices.

Earlier this month an armed man walked into the headquarters of the Arkansas Democratic Party and killed its chairman, a close ally Bill Clinton, the former US president, and his wife, Hillary. The shooter was later shot dead by police.

Five people died in 2001 after anthrax powder was mailed to US congressional offices and media outlets in the weeks following the September 11 attacks.

Earlier this month Bruce Ivins, a US scientist whom authorities said they were due to charge over the anthrax attacks, committed suicide.

His lawyer said he was innocent of the allegations.