A Russian submarine surfaced near Alaska on Thursday during a Russian war game exercise, US military officials said, AP reported.

It was unclear why it surfaced.

The North American Aerospace Defense Command and US Northern Command are closely monitoring the submarine, Northern Command spokesman Bill Lewis said.

“We have not received any requests for assistance from the Russian Navy or other mariners in the area,” Lewis said from Peterson Air Force Base in Colorado. “We always stand ready to assist those in distress.”

Lewis declined to provide further details about the submarine, including its proximity to Alaska. He only said it was operating in international waters near Alaska.

The Russian military exercise is taking place in international waters, well outside the US territorial sea, he said.