Hundreds of early ballots cast for the US presidential election have been burned in two suspected attacks in Washington and Oregon, exacerbating tensions ahead of next Tuesday’s knife-edge contest.

Police said on Monday that the fires in the two states were believed to be connected and that a vehicle involved had been identified, according to the Associated Press.

Firefighters went to the scene after smoke was reported coming from a ballot drop box in the city of Vancouver in Washington state at 6.30am on Monday, according to local media.

KATU, a local television channel, reported capturing footage of responders releasing a pile of burning ballots to the ground. The ballots continued to smolder after the flames had been doused.

Hundreds of ballots were believed to have been inside when smoke was reported billowing from the box, which had last been emptied at 8am on Sunday. KATU reported that only a few of the ballots deposited there after that had been saved.

The elections auditor for Clark county, the local authority administering the boxes, said voters who had cast their ballots into it after 11am could seek new voting documents at a link on the county’s election webpage.