COPENHAGEN, Denmark (AP) — The airport in Norway’s second-largest city was briefly closed Wednesday after at least one drone was spotted nearby by area residents, authorities said.

The first drone sighting at Bergen Airport was reported at 4:15 a.m., police spokesman Ørjan Djuvik said, adding that the Norwegian army and state-owned airports operator Avinor were informed. Police said no suspects had been identified.

Other drone sightings were later reported.

“There can also be observations that could be other phenomenon, for instance weather,” Djuvik said, but adding that “we are unsure whether it is one or more drones, but we are sure that there is at least one.”

Bergen Airport, which has 15 gates, was shut down at around 6:30 a.m. and reopened 2½ hours later. It's the second-largest airport in Norway and serves more than 6 million passengers a year, according to the airport’s website.

Numerous drone sightings have been reported near Norwegian offshore oil and gas platforms and infrastructure in recent weeks.

On Monday, Norwegian Justice Minister Emilie Enger Mehl asked people to be aware of suspicious activity and said that domestic security agency PST had received a number of new drone tips.

“I would say that it is important that we have vigilance in society for activity that may appear suspicious. This should be reported to the local police,” she said.

At least six Russian citizens have been detained over the past few weeks for flying drones or taking photographs of sensitive sites in Norway.

There is heightened security around key energy, internet and power infrastructure following last month’s underwater explosions that ruptured two natural gas pipelines in the Baltic Sea that were built to deliver Russian gas to Germany.

The blasts happened off both Sweden and Denmark — in international waters but within the countries’ exclusive economic zone. The damaged Nord Stream pipelines discharged huge amounts of methane, a potent greenhouse gas, into the air.