MANILA, May 29 (Reuters) - The coast guard of the United States, Japan and the Philippines will hold trilateral maritime exercise in the South China Sea this week, the first such manoeuvres between them as a time of growing concern about China's moves in the region.

The June 1 to 7 exercise in waters off Bataan province was as initiative of the United States and Japan, while Australia would join as an observer, said Philippine coast guard spokesman Armand Balilo said on Monday.

Four Philippine vessels and one each from the United States and Japan will participate in exercises designed to improve search and rescue collaboration and law enforcement, Balilo said.

The Philippines was approached by Japan and the United States about holding joint maritime exercises in February, the same month when Manila accused China of aggressive activities in the South China Sea, vast stretches of which Beijing claims as its territory.

"This is a usual routine activity among coast guard agencies," Balilo told a press conference.

"There is nothing wrong with holding exercises with your counterparts."

Japan, Australia and the United States have frequently condemned China's militarisation in the South China Sea and have sought to engage closer with U.S. ally the Philippines since Ferdinand Marcos Jr took over as president last year from pro-China predecessor Rodrigo Duterte.

Philippine ties with the United States have gained ground under Marcos, who has been increasingly vocal about China's conduct, including over its alleged use of a "military-grade laser" against a vessel supporting a navy food re-supply mission.

It has also complained about large numbers of suspected militia lingering near Philippine-held features in the disputed Spratly islands. China maintains the actions of its coast guard are legal and in its waters.

Balilo said the upcoming maritime exercise will include counter-piracy simulations, and possibly an interception exercise involving a vessel carrying weapons of mass destruction.