The Chinese Coast Guard on Friday accused the Philippines of illegally entering its waters, particularly in Ayungin Shoal, which they call Ren'ai Reef.

"On September 8, two replenishment ships and two coast guard vessels from the Philippines entered the adjacent waters of Ren'ai Reef in China's Nansha Islands without the permission of the Chinese government," an unnamed spokesperson of the CCG said in a statement.

According to the CCG, it "strictly" warned the Philippine vessels "in accordance with the law, tracked and monitored the vessels all the way, and effectively regulated them."

It also maintained its sovereignty over the Spratly Island or Nansha Qundao to China, as well as the Philippines' activities in the waters.

"China has indisputable sovereignty over the Nansha Islands, including the Ren'ai Reef, and its adjacent waters, and firmly opposes the illegal transportation of construction materials by the Philippines to the illegal grounded warship," it said

"The China Coast Guard will continue to carry out law enforcement activities in the sea areas under China's jurisdiction according to law," it added.

China claims the vast South China Sea, including the West Philippine Sea. Ayungin Shoal, also known as Second Thomas Shoal, is located 105 nautical miles west of Palawan and is part of the Philippines' exclusive economic zone.

In 2016, the Philippines won its arbitral case against the People's Republic of China in the Permanent Court of Arbitration—a landmark decision that Beijing continues to reject.