Beijing expressed opposition on Monday to Washington's emboldening Manila to infringe on China's sovereignty in the South China Sea, saying the attempt is doomed to fail.

Despite China's repeated warnings, the Philippines persisted in sending two ships into the waters of Ren'ai Reef in the Nansha Islands without authorization on Aug 5, carrying construction materials to reinforce an warship which has been illegally stranded in the reef since 1999, the Foreign Ministry said in an online statement.

Noting the move violated the Declaration on the Conduct of Parties in the South China Sea, a Foreign Ministry spokesperson said in the statement Chinese coast guard vessels stopped the Philippine ships in accordance with the law.

However, the United States State Department voiced support to the Philippines and criticized China, calling on Beijing to abide by the South China Sea arbitration.

The US statement attacks China's legitimate maritime law enforcement actions and endorses illegal provocations by the Philippines, which China firmly rejects, the Foreign Ministry said.

The US has been encouraging the Philippines to repair and reinforce its "grounded" warship, sending military aircraft to support the Philippines and frequently threatening China with the US-Philippines Mutual Defense Treaty, the spokesperson said.

Calling the South China Sea arbitration a political farce manipulated by the US under the guise of law, the spokesperson said it is null as it violated international laws including the United Nations Convention on the Law of the Sea.

China urges the US to stop using the South China Sea issue to sow discord and respect China's territorial sovereignty and maritime rights and interests, the spokesperson said.

The spokesperson also urged Washington to respect efforts from regional countries to maintain peace and stability in the area.