Deepening Chinese activities in the Arctic region could pave the way for a strengthened military presence, including the deployment of submarines to act as deterrents against nuclear attack, the Pentagon said in a report released on Thursday.

The assessment is included in the U.S. military’s annual report to Congress on China’s armed forces and follows Beijing’s publication of its first official Arctic policy white paper in June.

In that paper, China outlined plans to develop shipping lanes opened up by global warming to form a “Polar Silk Road” - building on President Xi Jinping’s signature Belt and Road Initiative.

China, despite being a non-Arctic state, is increasingly active in the polar region and became an observer member of the Arctic Council in 2013. That has prompted concerns from Arctic states over Beijing’s long-term strategic objectives, including possible military deployments.

U.S. Secretary of State Mike Pompeo will attend the meeting of the eight-nation Arctic Council in Rovaniemi, Finland, starting on Monday, which comes amid concerns over China’s increased commercial interests in the Arctic.

The Pentagon report noted that Denmark has expressed concern about China’s interest in Greenland, which has included proposals to establish a research station and a satellite ground station, renovate airports and expand mining.

“Civilian research could support a strengthened Chinese military presence in the Arctic Ocean, which could include deploying submarines to the region as a deterrent against nuclear attacks,” the report said.

The Pentagon report noted that China’s military has made modernizing its submarine fleet a high priority. China’s navy operates four nuclear-powered ballistic missile submarines, six nuclear-powered attack submarines and 50 conventionally powered attack submarines, the report said.

“The speed of growth of the submarine force has slowed and (it) will likely grow to between 65 and 70 submarines by 2020,” the report predicted.

The report said China had built six Jin-class submarines, with four operational and two under construction at Huludao Shipyard.

In a January report, the Pentagon’s Defense Intelligence Agency said the Chinese navy would need a minimum of five Jin-class submarines to maintain a continuous nuclear deterrence at sea.