A U.S. navy destroyer ship sailed within 12 nautical miles of an artificial island built by China in the South China Sea on Friday, as part of a “freedom of navigation" operation.

An official who spoke on the condition of anonymity said the area where the destroyer sailed — close to Mischief Reef in the Spratly Islands — has been a topic of dispute for China and its neighboring nations for years, the Strait Times reported. No other details regarding the operation were revealed.

“Freedom of navigation,” encoded as article 87(1)a of the 1982 United Nations Convention on the Law of the Sea, is a principle of customary international law that says operations carried out by a ship that bears the flag of a sovereign state, will not be interfered with by any other state, barring certain specified exceptions.

A news report of the U.S. navy’s mission came on the same day that the Chinese navy was supposed to carry out combat drills on the South China Sea.

Since the People’s Liberation Army Daily (the Chinese newspaper which reported the drills) did not elaborate on the specific timings of the drills or which part of the sea they would be carried out and which ships would take part in them, there is no way of telling if the drills were performed at the same time as the “freedom of navigation” operation. 

“This is a routine annual planned arrangement for the navy, the aim of which is to test and improve the military’s training level and to fully raise the ability to win. It is not aimed at any specific country or target,” the newspaper reported.

Combats drills are routine exercises carried out by the Chinese navy on various parts of the South China Sea, including those which are disputed. China has often been blamed of stirring up tension in the region by building man-made islands and constructing airstrips and other facilities there, Reuters reported.

The other nations which have conflicts with China regarding territory in the South China Sea are Brunei, Malaysia, Taiwan, Vietnam and the Philippines. However, a majority of the sea is claimed by China — an area which acts as a key trade route as well as is believed to hold large quantities of oil and natural gas.

Earlier this week, Taiwan's defense ministry said it shadowed a Chinese aircraft carrier group travelling in a southwesterly direction in the Taiwan Strait. Although the aircraft carrier was within the disputed stretch of the sea, the Taiwanese authorities did not raise a hue and cry about it because they thought it was part of China’s annual drills.

The first time that a U.S. destroyer carried out a “freedom of navigation" operation under President Donald Trump, was in May 2017. The guided-missile destroyer, the USS Dewey sailed within 12 nautical miles of the artificial Mischief Reef in the South China Sea's Spratly Islands at the time.

The Department of Defense did not confirm the reports of any such operation at the time. “U.S. forces operate in the Asia-Pacific region on a daily basis, including the South China Sea,” Defense Department spokesman Jamie Davis said in a statement.

US Destroyer A U.S. navy destroyer ship sailed within 12 nautical miles of an artificial island built by China in the South China Sea on Friday, as part of a “freedom of navigation" operation. In this photo, provided by the U.S. Navy, the Arleigh Burke class guided-missile destroyer USS Fitzgerald (DDG 62) is on patrol on Sept. 8, 2014. Photo: Getty Images/ Mass Communication Specialist Seaman David Flewellyn