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

Firefighters responded to the scene after smoke billowed from a ballot box in the city of Vancouver, Washington, at 6:30 a.m. Monday, according to local media.

KATU, a local television station, reported that footage was captured of emergency workers clearing a pile of burning ballots at the site. The ballots continued to smolder after the flames were extinguished.

Hundreds of ballots were believed to have been received when smoke was reported coming from the ballot box, which was last emptied at 8am on Sunday. KATU reported that only a few of the ballots deposited there afterward had been retained.

The Clark County Election Auditor, the local authority that administers the polls, said voters who cast ballots after 11 a.m. could request new voting documents through a link on the county’s elections page.

The fire was reported after a similar incident in nearby Portland, Oregon, where police said an incendiary device was detonated in a ballot box near a building housing the Multnomah County Elections Department.

Security personnel extinguished the fire before police arrived. The device was deactivated and removed by the local bomb squad.

The incidents came days after a US Postal Service mailbox containing a small number of ballots was set on fire in Phoenix, Arizona, last Thursday.

Police have arrested a 35-year-old man who they say committed the crime while in custody. They also said he told them his actions were not politically motivated and that he entered office with the intention of getting himself arrested.

The Guardian has reported that far-right election denial groups supporting Donald Trump have been monitoring election mailboxes as part of their activities ahead of next week’s election, as officials brace for disruption and challenges to voting tallies.