China's military said on Saturday it monitored and tracked a joint patrol organized by the Philippines in the disputed South China Sea on October 30 and October 31.
Washington and Manila have beefed up military cooperation, unveiling plans on Friday to form a new joint task force for areas including the South China Sea, a conduit for more than $3 trillion of annual ship-borne commerce.
Tian Junli, a spokesperson of the Southern Theater Command of the Chinese People's Liberation Army, said the patrol, with unnamed partners, "seriously undermined regional peace and stability".
He called the Philippines "a troublemaker" in the region.
"The theater command forces remain on high alert and will resolutely safeguard national territorial sovereignty and maritime rights and interests," Tian added in a statement.
The Philippine embassy in Beijing did not immediately respond to an emailed request for comment.
The armed forces of Australia, New Zealand, the Philippines and the United States held a drill in the South China Sea on October 30 and 31.
The US 7th Fleet said the exercise aimed to demonstrate "a collective commitment to strengthen regional and international cooperation in support of a free and open Indo-Pacific".
China claims almost the entire South China Sea, including parts claimed by Brunei, Indonesia, Malaysia, the Philippines and Vietnam.
In 2016 the Permanent Court of Arbitration in the Hague ruled that China's claims were not supported by international law, a decision Beijing rejects.
