NATO is increasingly concerned about China’s shipping on Russia’s Northern Sea route, and the possibility that its commercial and scientific interests could be a precursor to a Chinese military presence in the Arctic, the alliance’s senior military officer said.

“We know there are military scientists on board these ships,” Admiral Rob Bauer, who chairs NATO’s Military Committee, said in an interview in Iceland on Saturday. “They haven’t said they won’t go there militarily.”

NATO is concerned about China’s aims given its increasingly close ties with Russia, including cooperation on energy and transportation that have caused a surge in Russian crude shipments to China through Arctic waters.

Bauer’s remarks followed a speech at the Arctic Circle Assembly conference in Reykjavik, during which he raised concern about stronger Russia-China ties. While Russia’s intentions in the Arctic have become increasingly clear, “China’s intentions for the region remain opaque,” he told the audience.

Gao Feng, China’s special representative for Arctic affairs, didn’t directly address Bauer’s comments in his conference speech.

“Tension in the Arctic will not only make it difficult to carry out our cooperation, but also will affect global stability,” Gao said. “China and other Asian countries can play important roles in the protection, research, development and governance of the Arctic.”

Read more: NATO-China Tension Over Ukraine Flares at Conference in Iceland

In what’s seen as a first step toward regular small-scale container shipping via the Northern Sea route, a Chinese freighter used the passage for a three-month round trip from the Baltic Sea to China, while Russia’s realigning of commercial ties after its invasion of Ukraine has boosted oil shipments to China.

“We cannot be naive and expect these new routes to solely be used by commercial vessels,” Bauer told the conference.

At the group’s meeting last year, Bauer drew the ire of a Chinese diplomat in the audience, who accused him of arrogance. In return, Bauer asked why China hadn’t condemned Russia’s attack on Ukraine.

Russian Threat

Russia has been investing heavily in Arctic air bases and other infrastructure. While budgetary restrictions related to the war in Ukraine have delayed the delivery of new icebreakers, its substantial air and naval assets in the Arctic remain intact, Bauer said in the interview.

NATO must be prepared for conflict in the region given Russia’s ability to operate in the Arctic, Bauer told the conference. Even so, NATO isn’t worried about the Arctic becoming a hotspot “immediately,” he said.

--With assistance from Ragnhildur Sigurdardottir.