Swedish police have launched an investigation into suspected sabotage after the country’s largest airport temporarily suspended all flights following four drone sightings overnight on Sunday, Reuters news agency reported, citing local police.

Air traffic at Stockholm’s Arlanda airport was suspended at around 11pm and resumed at 1:30am (local time), according to the police statement.

“We suspect that this was a deliberate act, but I cannot answer what the purpose was. Due to the confidentiality of the investigation, we do not want to go into detail about what type of drones were spotted and by whom,” police spokesman Daniel Wikdal said.

Sweden’s intelligence agency last month warned of an increased threat of Russian sabotage attacks in the country, after arms manufacturers reported a rise in break-in attempts and surveillance by drones.

Swedish Civil Aviation Authority spokeswoman Cecilia Begström said that four drones were spotted over the airport. Their model and country of origin is unknown, while the police did not report on whether the drones were destroyed or whether they left the airport on their own.

The drone incident in Stockholm comes a day after NATO members Romania and Latvia reported instances of Russian drones crashing on their territory, prompting NATO officials to call for measures to act jointly to counter Russia air incursions.

Latvia, which borders Russia and its ally Belarus, has not previously reported any incidents of Russian drones breaching the country’s airspace before.

Romania shares a 650-kilometre border with Ukraine and has had Russian drone fragments stray into its territory repeatedly over the past year. Romanian territory lies a few hundred metres from Ukraine’s Danube River ports, which are frequent Russian targets.