A high mobility artillery rocket system (HIMARS) is fired during a live fire exercise in the annual joint military exercises between U.S. and Philippine troops called "Balikatan" or shoulder-to-shoulder, held in San Antonio, Zambales province, Philippines, April 26, 2023. REUTERS/Eloisa Lopez

SAN ANTONIO, Philippines (Reuters) - U.S. and Philippine armed forces unleashed a volley of missiles on a mock enemy warship in the South China Sea on Wednesday, in a show of military power and a strengthening alliance at a time of rising regional tension.

Philippine President Ferdinand Marcos Jr watched from a four-storey tower as a high mobility artillery rocket system (HIMARS) fired rockets at a decommissioned navy corvette in the waters just a few miles off its western province of Zambales.

It was the first time the annual U.S.-Philippines "Balikatan" exercises featured live fire drills at sea, at a location just a few hours by boat from one of the world's most contested maritime features, the Scarborough Shoal, which China has occupied for more than a decade.

Military from both countries have said the exercises across the Philippines, which include sites facing Taiwan, were not targeted at any country.

This year's war games end on Friday and with 17,000 troops are the biggest edition yet, in a sign of rekindled ties between the defence treaty partners after a period of strain under the previous, anti-U.S. president, Rodrigo Duterte.

"The alliance is alive," said Colonel Michael Logico, a Philippine spokesperson for Balikatan.

Marcos, who was joined by his defence and armed forces chiefs, boarded and inspected a HIMARS cab displayed at the San Antonio naval base before observing the exercise through binoculars and on a TV screen.

He smiled the moment the first missile was fired, leaving a trail of smoke in the sky, part of a coordinated coastal, inland artillery bombardment and airstrike that obliterated the mock enemy target.

"The president came away with a deeper appreciation for joint and combined operations," spokesperson Logico added.

Marcos is due to meet U.S. counterpart Joe Biden in Washington next week and plans to discuss a 1951 Mutual Defense Treaty that he feels must evolve in the face of "heating up tensions".

China has criticised the military exercises, which come against the backdrop of what the Philippines calls "aggressive" Chinese actions in the South China Sea.

Beijing has said the U.S.-Philippines alliance must not interfere in sea disputes or harm China's territorial sovereignty and security interests. The Philippines and United States say their activities are for defence purposes only.

"There's nothing provocative about training to defend from a nation's sovereign territory," Major General Joseph Ryan, commanding general of the U.S Army's 25th Infantry Division, told reporters.

"It is every sovereign nation's right to defend their space."

(Additional reporting by Neil Jerome Morales; Editing by Martin Petty)