Chinese navy warships arrive at the seaport of Port Sudan August 25, 2015. The Chinese navy warships are at the seaport of Port Sudan as part of military cooperation between the two countries, according to the Sudanese Armed Forces (SAF). REUTERS/Stringer (Mohamed Nureldin Abdallah/Reuters)

A group of Chinese naval vessels transited U.S. territorial waters near Alaska this week, a Pentagon official said Friday, in an unusual maneuver that underscores the potential for increased U.S.-Chinese friction at sea.

A U.S. military official, who spoke on the condition of anonymity to discuss details of the Chinese naval movements, said the group of five Chinese vessels had passed within about 12 nautical miles of the Aleutian Islands after a joint Russian-Chinese military exercise.

The ships did not violate international law, which allows countries to transit other nations’ seas under what is called “innocent passage,” the official said. He likened China’s movement through U.S. waters off Alaska to the activities of U.S. ships in the Strait of Hormuz, off the coast of Iran.

The official said the flotilla, which included three surface combatant ships, one amphibious ship and a supply ship, now appeared to be heading back to China. “By all accounts and by all indications, they’re going home,” he said.

Navy Cmdr. Bill Urban, a spokesman at the Pentagon, said the ships continued out to sea into the Pacific Ocean after transiting the Aleutian Island chain. Defense officials said they did not move any Navy ships in response to China’s vessels. The Defense Department does not plan to provide additional updates on the location of the ships, indicating that Defense officials probably consider the issue resolved.

The Chinese ships, whose passage through U.S. waters was first reported by the Wall Street Journal, were spotted as President Obama concluded a high-profile visit to Alaska.

“This is clearly a signal,” said David Titley, a retired rear admiral who is a professor at Penn State University and an adjunct senior fellow at the Center for a New American Security. Of what, Titley said, it’s difficult to say, but he suggested that China may be seeking to establish itself as a player in growing commercial activity in the Arctic.

Titley said the incident — and especially the lack of a dramatic U.S. response — could also have a softening effect on China’s position on maritime disputes in the South China Sea.

As Chinese military spending grows, U.S. officials have criticized China’s aggressive stance in the South China Sea and its attempt to strengthen territorial claims by building up islands. Titley said a shrug by U.S. officials about this week’s activity in the Bering Sea may lead leaders in Beijing to ratchet down their own response to foreign presence off China.

“This is how mature superpowers operate,” he said. “You don’t go to general quarters every time there’s a ship operating legally.”

With China expanding its military, the United States appears willing to let it spread its wings as long as it doesn’t cross legal lines.

“China is a global navy, and we encourage them and other international navies to operate in international waters as long as they adhere to safe and professional standards and maritime laws of the sea,” said Navy Cmdr. William Marks, a Navy spokesman at the Pentagon.

The United States has operated in international waters off China’s coast for years, especially in the South China Sea, where tensions can sometimes erupt between China and neighboring countries. In the past two months alone, at least three U.S. warships have spent time there: the USS Fort Worth, a littoral combat ship; the USS Lassen, a destroyer; and the USS Germantown, an amphibious dock landing ship. The Fort Worth participated in an exercise with Malaysia, while the Lassen worked with Singapore and the Germantown worked with Indonesia, Marks said.

There are 10 Pacific Fleet ships underway in the Western Pacific, said Lt. Cmdr. Clay Doss, a Navy spokesman. None are making port visits in China now, but several have this year in places such as Zhanjiang and Hong Kong.

Pacific Fleet ships routinely use international waters in the East China Sea, South China and other parts of the Pacific. Maritime patrol aircraft, such as the P-8 Poseidon, use international airspace overhead in similar fashion, Doss said.

The USS Preble, a destroyer, and the USS Chancellorsville, a cruiser, were in the South China Sea last month, Doss said.

“As we’ve said before, these patrols are not aimed at any particular navy, but are about promoting freedom of the seas for all navies in international waters,” he said.