Washington State Governor Jay Inslee has activated and put part of the National Guard on standby in case of possible election disturbances.

"This is purely a precautionary measure," said a statement released by the governor's office.

Earlier in the week, a ballot box containing early voter ballots burned in Vancouver, Washington state, destroying hundreds of ballots. A similar incident occurred at the same time in Portland - Oregon.

The governor, who is a Democrat, responded to the incidents as well as warnings from the US Department of Homeland Security about threats to election infrastructure.

In the United States, the National Guard is a state military organization organized by individual federal states and reporting to governors and the president, and its members are often deployed in the event of natural disasters and other emergencies.

It consists of units of the ground army and the air force. In Washington state, the Guard will be on standby for four days, starting Monday, and will end duty after midnight Thursday. Election day is Tuesday, but there are already tens of millions of early votes.