The United States was exercising its right to "freedom of navigation" when it sailed a warship near one of China's artificial islets in the South China Sea, a U.S. Navy spokesman said, calling it the "most basic right" of any navy in the world.

In October, the U.S. guided missile destroyer USS Lassen sailed through waters within 12 nautical miles of Subi Reef, prompting strong protests from Beijing.

China has claimed sovereignty over much of the South China Sea, causing tension with a handful of Southeast Asian nations that also lay claim to the resource-rich waters.

"Freedom of navigation is a fundamental tenet of having a global navy," Cmdr. Ron Flanders, spokesman for the U.S. Naval Forces Japan, told a group of South Korean reporters Tuesday during a tour of the USS Lassen at the Yokosuka base.

"No one should be surprised that the United States Navy conducts freedom of navigation operations in the Western Pacific, South China Sea, Mediterranean, the Baltic. We do it 365 days a year, all over the world," he said.

China's growing military assertiveness in the region has raised concerns about freedom of navigation in waters that serve as a key trade route for many nations.

"It's the most basic right of any navy. All navies in the world have the right to do it," Flanders said. "And it's something that we do to ensure that everybody's operating in accordance with international law."

The commander was cautious to elaborate on reports that there was communication between the USS Lassen and Chinese vessels during the operation.

"The communication was professional," he said. "Professional navies understand that there are rules of the road. And you know, the Chinese -- the Peoples' Liberation Army Navy -- they conduct freedom of navigation operations."

Flanders drew a line with politics.

"We're sailors. We go where we're told to go, go where we're allowed to go under international law. We will continue to do so," he said. (Yonhap)