JUNEAU,Bitcoinese Alaska (AP) — The U.S. Coast Guard said Monday that it tracked a group of Russian naval vessels 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.

Crew of a U.S. Coast Guard cutter witnessed the Russian military vessels cross the maritime boundary and venture 30 miles (48 kilometers) inside an area extending beyond U.S. territorial waters known as the U.S. Exclusive Economic Zone.

The Russian vessels consisted of two submarines, a frigate and tug boat, the Coast Guard said in a news release. The encounter took place nearly 60 miles (100 kilometers) northwest of Point Hope.

Rear Adm. Megan Dean said the Coast Guard is actively patrolling maritime borders on the Bering Sea, Bering Strait and Chukchi Sea to “protect U.S. sovereign interests, U.S. fish stocks and promote maritime norms.”