(Adds Chinese foreign ministry response in paras 2-3)

MANILA, Nov 26 (Reuters) - Two Chinese fighter jets were monitored "orbiting" a Philippine aircraft participating in patrols with Australia in the South China Sea but did not cause any untoward incident, Manila said on Sunday.

China's foreign ministry said Monday it was "unaware of the circumstances".

"I want to emphasise that if China's territorial sovereignty and maritime rights and interests are threatened or challenged, China will definitely have a firm response," the spokesperson's office said in a faxed message to Reuters.

The militaries of the Philippines and Australia carried out a second day of sea and air exercises in the Southeast Asian country's exclusive economic zone, days after Manila held patrols with the U.S. as Pacific nations warily eye an increasingly assertive China.

China claims almost the entire South China Sea, a conduit for more than $3 trillion of annual ship-borne commerce, including parts claimed by the Philippines, Vietnam, Indonesia, Malaysia and Brunei. The Permanent Court of Arbitration in 2016 said China's claims had no legal basis.

The Philippines is ramping up efforts to counter what it describes as China's "aggressive activities" in the South China Sea, which has also become a flashpoint for Chinese and U.S. tensions around naval operations.

China has accused the Philippines of enlisting "foreign forces" to patrol the South China Sea and stirring up trouble.

"It was confirmed as per reports received that two Chinese fighter jets were monitored orbiting the Philippines' A-29B Super Tucano at the vicinity of Hubo Reef in the West Philippine Sea," Xerxes Trinidad, chief of the Philippine military's public affairs office said.

The West Philippine Sea is Manila's term for waters in the South China Sea that fall within its exclusive economic zone.

The Chinese aircraft continued on its flight route without further incident, Trinidad said.

Philippine military chief Romeo Brawner said the nation was well its rights to conduct joint patrols with allies to promote the "rule-based international order."

Participating in the joint drills were two Philippine navy vessels and five Philippine surveillance aircraft, and Australia's frigate Toowoomba and P8-A maritime surveillance aircraft. (Reporting by Karen Lema; Additional reporting by Laurie Chen in Beijing; Editing by William Mallard and Toby Chopra)