Amid ongoing tensions between the United States and China, the US Navy said it had allowed a destroyer to sail through disputed waters in the South China Sea on Monday. With the “999,” the US “guaranteed rights, freedoms and the lawful use of the sea,” the Navy said on Monday. Beijing described the US action as “illegal.”

According to the US Navy, the guided missile destroyer USS Milius also passed the Spratly Islands, which have been disputed for years between China, the Philippines and other neighboring countries. The warship “exercised the rights and freedoms of navigation in the South China Sea in the vicinity of the Spratly Islands in accordance with international law,” it said.

After the passage, the USS Milius left the area and continued its activity in the South China Sea, the US Navy said. Accordingly, the ship moved within twelve nautical miles of the Mischief Reef, which belongs to the Spratly Islands.

China described the US actions as “illegal”. The USS Milius “illegally entered the waters of China’s Nansha Islands adjacent to the Meiji Reef without the consent of the Chinese government,” a spokesman for the People’s Liberation Army’s Southern Territorial Command said on Monday. The Chinese Air Force “tracked and monitored” the ship.

China claims practically the entire South China Sea for itself. However, Brunei, Malaysia, the Philippines and Vietnam also lay claim to parts of the sea area, which is of enormous strategic and economic importance for the neighboring countries. Beijing is also fueling the territorial conflict by piling up artificial islands and building military installations there. Neighboring countries also accuse Chinese ships of harassing fishing boats.

The US considers Beijing’s territorial claims in the South China Sea to be illegal and uses military vessels to enter the area to assert freedom of navigation in international waters.

Tensions between the United States and China have escalated in recent days. For the third day in a row, China held military maneuvers off Taiwan, some 1,300 kilometers from the Spratly Islands, during which Beijing’s military said it practiced “locking down” Taiwan.