China claims that the Philippines is using foreign forces to cause unrest in the South China Sea.

The Philippines has enlisted “foreign forces” to patrol the South China Sea and has been stirring up trouble, China’s military said on Thursday, referring to joint patrols this week by Philippine and U.S. forces.

The Chinese military will maintain high vigilance, resolutely defend sovereignty and maritime rights and interests, and resolutely safeguard peace and stability in the South China Sea, China’s military said.

“The Philippines enlisted forces out of the region to patrol … stirred up trouble and engaged in hype, undermining regional peace and stability,” the southern theatre command of the Chinese military said.

Philippine officials have said their military and the U.S. launched joint patrols on Tuesday in waters near Taiwan, a democratically governed island that China claims as its own, raising the possibility of further tensions with China.

Relations have soured between China and the Philippines over the South China Sea under Philippine President Ferdinand Marcos Jr as he pivots towards closer ties with the U.S., which supports the Southeast Asian nation in its maritime disputes with China.

A Chinese foreign ministry spokesperson said on Wednesday China had warned the U.S. and the Philippines in connection with their patrols.

“China has made clear its position to the Philippines and the U.S. that the Philippine-U.S. joint patrols must not undermine China’s territorial sovereignty and maritime rights and interests,” spokesperson Mao Ning said.

A Chinese navy ship shadowed three U.S. and Philippine warships conducting joint patrols on Thursday, the Philippine armed forces chief, Romeo Brawner, told reporters.

The incident occurred at around 10:15 a.m. near the platform of a natural gas field 27 nautical miles off Palawan province, Brawner said. “There was no challenge, no dangerous manoeuvres.”

The Philippines achieved its goal of closely operating with its ally, the United States, and there was no untoward incident, Brawner added.

The Philippine foreign ministry and the national security adviser’s office did not immediately respond to a request for comment.