The U.S. Coast Guard said Monday that it tracked a group of Russian naval vessels, including two submarines, as they crossed into U.S. waters off Alaska in an apparent effort to avoid sea ice, a move that is permitted under international rules and customs.

The U.S. military has moved about 130 soldiers along with mobile rocket launchers to a desolate island in Alaska amid a recent increase in Russian military planes and vessels approaching the state.

The North American Aerospace Defense Command (NORAD) detected and tracked several Russian aircraft in the Alaska area over the last week. Two aircraft were detected Sunday and two on Saturday, according to NORAD.