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.

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 grounds. 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 web page.

“There is absolutely zero place in our democracy for political violence or interference against our fellow citizens, election workers, or voting infrastructure … Our right to vote needs to be protected under all circumstances. We can’t yield to intimidation, and we must continue to stand up against unpatriotic acts such as this one,” said local congresswoman Marie Gluesenkamp Perez.

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She requested law enforcement be in place overnight at all ballot drop boxes in the county until election day, saying: “South-west Washington cannot risk a single vote being lost to arson and political violence.”

The fire was reported after a similar incident in nearby Portland in Oregon, where police say an incendiary device was set off inside a ballot drop box close to a building hosting the Multnomah county elections division.

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

The US Department of Homeland Security (DHS) warned of ballot drop box destruction in a September memo obtained by Property of the People, a public records watchdog group. The agency said in an intelligence brief that election infrastructure will be seen as an “attractive target for some domestic violent extremists”, with drop boxes as a “soft target” because they’re more accessible.

Social media posters in forums frequented by extremists have shared ideas for attacked drop boxes, the agency said, including “road flares, fireworks, petroleum fuel, linseed oil and white phosphorous, cement or expanding foam, bleach or other chemicals, and farm machinery”. Other methods could include putting up fake signs to claim a drop box is out of order, putting up decoy drop boxes or putting “timed explosives” into drop boxes. They have also discussed ways to avoid law enforcement detection.

“Damaged ballot drop boxes could temporarily decrease voting opportunities and accessibility and intimidate voters from casting votes if safety concerns arise in the vicinity of a targeted or damaged ballot drop box,” DHS wrote in the intelligence brief. “Successful ballot drop box destruction could inspire others with related grievances to conduct similar actions.”

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

Police arrested a 35-year-old man who they said admitted to the crime while he was in custody. They also said he had told them his actions had not been politically motivated and he had committed the office with the purpose of getting himself arrested.

The Guardian has reported that far-right election denial groups supporting Donald Trump have been monitoring election drop boxes as part of their activity in the run-up to next week’s poll, when officials are bracing themselves for disruption and challenges to the vote tallies.