Russian military activities and infrastructure build-ups in the Arctic are not going unnoticed, said the Pentagon press secretary, whose respective statement has been posted on the official website of the US Department of Defense.

"Without getting into specific intelligence assessments, obviously we're monitoring it very closely," said John Kirby during a briefing Monday at the Pentagon.

As ice melts in the Arctic, new options for transiting the Arctic open up—and also remove natural barriers that Russia once relied on to protect its own interests there.

Now, it is seeking to bolster its security through the refurbishing of Soviet-era airfields, the expansion of its network of air and coastal defense missile systems and the strengthening of its anti-access and area-denial capabilities.

But the US has its own interests in the Arctic as well, Kirby said.

"[We] obviously recognize that the region is key terrain that's vital to our own homeland defense and as a potential strategic corridor between the Indo-Pacific, Europe and the homeland—which would make it vulnerable to expanded competition," said the Pentagon press secretary. "We're committed to protecting our US national security interests in the Arctic by upholding a rules-based order in the region, particularly through our network of Arctic allies and partners who share the same deep mutual interests that we do."