(Adds Chinese Defense Ministry comment)
WASHINGTON, Sept 3 (Reuters) - Five Chinese Navy ships sighted in the Bering Sea off Alaska during a visit to the region by U.S. President Barack Obama have begun their "return transit," the U.S. Navy's top uniformed officer told Reuters on Thursday.
Chief of Naval Operations Admiral Jonathan Greenert said he did not view the incident, an apparent first for China's military, as unexpected or alarming.
"They already had one of their icebreakers up in that area, and they weren't that far away with an exercise, and they've already started their return transit," he told Reuters in an interview.
Greenert said the ships had been seen in the Bering Sea, close to some Alaskan atolls, on Wednesday, but gave no further details.
The Pentagon confirmed on Wednesday the ships had been sailing in international waters.
China's Defense Ministry, in a statement sent to Reuters on Thursday, said the ships were in the Bering Sea as part of a routine drill following the completion of military exercises with Russia.
"This is a routine arrangement as part of annual plans, and is not aimed at any set country or goal," it said, without providing any other details.
China has ramped up defense spending to modernize its forces and develop a navy capable of defending its growing interests as the world's second-largest economy.
At a regular State Department briefing in Washington on Thursday, spokesman Mark Toner said: "This is certainly the first time we have observed Chinese navy ships in the Bering Sea, but that said, we do certainly respect the freedom of all nations to operate military vessels in international waters in accordance with international law."
Pentagon spokesman Commander Bill Urban said the Chinese ships were now south of the Aleutian Islands, a chain of islands bounding the south side of the Bering Sea. He had no additional information.
U.S. defense officials said the Pentagon was continuing to monitor the movement of the ships, which appeared to be heading away from the region. (Reporting by Andrea Shalal; Additional reporting by Ben Blanchard in BEIJING; Editing by Peter Cooney and Dean Yates)