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.

Crew of the U.S. Coast Guard cutter Stratton witnessed the Russian military vessels cross the maritime boundary and venture 30 miles inside an area extending beyond U.S. territorial waters known as the U.S. Exclusive Economic Zone, the Coast Guard said in a news release.

The Russian vessels consisted of two submarines, a frigate and tug boat, the Coast Guard said. The encounter took place nearly 60 miles northwest of Point Hope.

"We are actively patrolling our maritime border in the Bering Sea, Bering Strait, and Chukchi Sea, with our largest and most capable cutters and aircraft, to protect U.S. sovereign interests, U.S. fish stocks, and to promote international maritime norms," said Rear Adm. Megan Dean. "Coast Guard Cutter Stratton ensured there were no disruptions to U.S. interests."

The Coast Guard said the Stratton, a 418-foot legend class national security cutter, is "patrolling under Operation Frontier Sentinel, an operation designed to meet presence with presence when strategic competitors operate in and around U.S. waters," the Coast Guard said.

There have been several similar encounters in recent months. Last month, a U.S. Coast Guard cutter on routine patrol around Alaska's Aleutian Islands came across a Russian ship in international waters but within the U.S. exclusive economic zone.

On July 6, the Coast Guard while on patrol spotted four Chinese military ships north of the Amchitka Pass in the Aleutian Islands in international waters, but also within the U.S. exclusive economic zone, officials said.

And on July 24, the U.S. military intercepted two Russian bombers and two Chinese bombers flying together in the Alaska Air Defense Identification Zone. The bombers were intercepted by U.S. and Canadian fighter jets and were not seen as a threat, according to North American Aerospace Defense Command.

However, a U.S. defense official told CBS News this was the first time that Russian and Chinese aircraft had jointly entered the Alaska ADIZ. It marked a new show of expanding military cooperation that U.S. Defense Secretary Lloyd Austin said at the time raises concerns.

While the Alaska ADIZ is considered part of international airspace, it is defined as an area where sovereign U.S. airspace ends but "that requires the ready identification of all aircraft in the interest of national security," according to NORAD.