Norwegian police said on Monday they arrested four Russians suspected of violating a photography ban, as the country fears possible sabotage at critical infrastructure sites.
Norway is on high alert following several reports of mysterious drone sightings close to offshore oil and gas drilling platforms run by the major energy producer.
Monday's announcement came just days after Norwegian police arrested two Russians in two separate incidents last week. They were accused of illegally flying drones and taking photos or videos.
Officers arrested the four in northern Norway in a car with Russian licence plates last Thursday and a day later placed them in custody for a week, regional police said in a statement.
Police have not disclosed what the four Russians -- three men and one woman -- were interested in but they had taken photos of objects covered by a photography ban.
The four, in their late 20s, had arrived in Norway from Finland in late September or early October.
They were arrested with a "substantial" amount of photos, but have denied any wrongdoing and have claimed they were just tourists, police official Gaute Rydmark told Norwegian television TV2.
Norway's Justice Minister Emilie Enger Mehl refused to comment on the matter, but said "there is heightened pressure on Norway, from an intelligence point of view".
Police said the four were not in possession of any drones, unlike the other two Russians arrested last week, also in the country's north.
Norway along with several other Western nations, has forbidden Russians and Russian entities from flying over its territory following Russia's invasion of Ukraine.
Breaking that ban is punishable by a three-year prison term.
Norway has also beefed up security at its offshore oil and gas drilling platforms after the drone sightings and last month's Nord Stream gas pipeline blasts in the Baltic Sea widely assumed to be the result of sabotage.
The Scandinavian country has overtaken Russia as the main supplier of natural gas to Western Europe after the invasion led to a cut in energy imports from Moscow.
On Sunday, the airspace around Norway's oil capital Stavanger was briefly closed after a drone was observed in the southwest region.
