The move is seen as one that could anger Beijing at a time of tense relations between the world’s two biggest economies.

The South China Sea is one of a growing number of flashpoints in the U.S.-China relationship, which also include a trade war, U.S. sanctions and Taiwan.

President Donald Trump dramatically increased pressure on China to reach a trade deal by announcing on Sunday he would hike U.S. tariffs on $200 billion worth of Chinese goods this week and target hundreds of billions more soon.

The U.S. guided-missile destroyers Preble and Chung Hoon traveled within 12 nautical miles of Gaven and Johnson Reefs in the Spratly Islands, a U.S. military spokesman told Reuters.

Commander Clay Doss, a spokesman for the Seventh Fleet, said that the "innocent passage" was "to challenge excessive maritime claims and preserve access to the waterways as governed by international law."

The operation was first reported by Reuters.

The U.S. military has a long-standing position that its operations are carried out throughout the world, including in areas claimed by allies, and that they are separate from political considerations.

The operation was the latest attempt to counter what Washington sees as Beijing’s efforts to limit freedom of navigation in the strategic waters, where Chinese, Japanese and some Southeast Asian navies operate.

China claims almost all of the strategic South China Sea and frequently lambastes the United States and its allies over naval operations near Chinese-occupied islands.

The Philippines, Brunei, Malaysia, Indonesia and Taiwan have competing claims in the region. Vietnam, which calls the waters the East Sea, has repeatedly asserted its sovereignty over the waters.

China and the United States have repeatedly traded barbs in the past over what Washington says is Beijing’s militarization of the South China Sea by building military installations on artificial islands and reefs.

China defends its construction as necessary for self-defense and says it is the United States that is responsible for ratcheting up tensions in the region by sending warships and military planes close to islands Beijing claims.

Last month, China’s navy chief said freedom of navigation should not be used to infringe upon the rights of other countries.

The freedom of navigation operation comes weeks after a major naval parade marking 70 years since the founding of the Chinese navy. The United States sent only a low level delegation to the Chinese navy anniversary events.