The FBI is investigating after two ballot drop boxes went up in flames in the Portland, Oregon, area early on Monday.

Hundreds of ballots were burned in Vancouver, a small city in Washington state less than 10 miles (16km) from Portland, with local media reporting that a device was stuck to the outside of a ballot drop box.

Meanwhile in the south-east part of Portland itself an incendiary device was placed inside a drop box, causing a small fire that damaged three ballots, according to officials.

Police said during a news conference on Monday that the two incidents were connected, and are "very similar" to an unsuccessful attempt to light another Vancouver ballot drop box on fire on 8 October.

Dedicated ballot drop boxes are used in a number of US states and cities to allow voters to submit their ballots early without having to wait in a queue on Election Day.

In September, the US Department of Homeland Security warned that some social media users have been promoting the destruction and sabotage of the boxes ahead of the election.

Ballots were last picked up from the Vancouver box on Saturday morning, and local officials said those who had deposited ballots since then should contact the local elections office.

Greg Kimsey, the local official overseeing elections, said the damaged box has been replaced, and officials are attempting to identify as many ballots as possible that were damaged in the fire.

Local police will increase their patrols around ballot drop boxes, he told the BBC.

“We hope these are isolated incidents that don't occur again,” Kimsey said.

Authorities in Portland said they were attempting to contact the voters who cast the three ballots damaged in that incident so that they can be issued replacements.

Police also released photos of a car they said was linked to the incidents, which they described as a dark-coloured Volvo.

Oregon and Washington are safe Democratic states in the presidential race, but the contest for a congressional seat is close in Vancouver – not to be confused with the much larger Canadian city.

Washington’s 3rd District includes Vancouver and a huge and largely rural section of the southern part of the state.

The seat is currently held by Democrat Marie Gluesenkamp Perez , who eked out a victory over Republican Joe Kent by less than 1% of the vote in 2022

Gluesenkamp Perez and Kent are headed to a rematch in next week’s election.

There was no word from police on who might have planted the devices that caused the ballot-box fires. The BBC has contacted authorities in both cities for comment.

The chief elections official in Washington, Secretary of State Steven Hobbs, said in a statement: “We take the safety of our election workers seriously and will not tolerate threats or acts of violence that seek to undermine the democratic process.”

Last week a man in Phoenix, Arizona, was charged with setting a US post box on fire that contained around 20 ballots, court records show.

The man told police his actions were not politically motivated, and that he was homeless and intended to commit a minor crime so that he would be sent to jail.

The Phoenix box was for general post, while the boxes set alight in Portland and Vancouver were only used for ballots.

With reporting by Sam Cabral