The Philippines and the United States launched the largest-ever joint military drills in the archipelago nation on Monday, signalling deepening defence ties as fresh tensions surface in the disputed South China Sea.

The war games are the last under outgoing President Rodrigo Duterte, who previously threatened to cancel exercises and axe a key military deal with longtime ally the United States as he pivoted towards China.

Nearly 9,000 Filipino and American soldiers will take part in the 12-day training event across the main island of Luzon, which is usually an annual affair but was cancelled or curtailed during the pandemic.

Philippine military chief General Andres Centino said at the opening ceremony in Manila that the largest round of the Balikatan war games reflected the "deepening alliance" between the two countries.

US Major General Jay Bargeron said the "friendship and trust" between their respective armed forces would allow them to "succeed together across the entire spectrum of military operations".

The exercises will cover maritime security, amphibious operation, live-fire training, counterterrorism, and humanitarian assistance and disaster relief.

Recent manoeuvres between the two countries focused on potential conflict in the South China Sea, which Beijing claims almost in its entirety.

Since taking power in 2016 Duterte has moved closer to China, but has faced pushback from the Philippine public and concern in the military wary of its territorial ambitions in the waters.

Trillions of dollars in trade pass through the strategic sea and it is thought to contain rich petroleum deposits, making it a frequent source of regional friction.

China has ignored a 2016 ruling by The Hague-based Permanent Court of Arbitration that its historical claim is without basis.

It has reinforced its stance by building artificial islands over some contested reefs and installing weapons on them.

Tensions spike

The future of the war games was thrown into doubt after Duterte said in February 2020 he planned to axe the Visiting Forces Agreement, which provides the legal framework for the US to hold joint military exercises and operations in the Philippines.

But he walked back on the decision last July, as tensions between Manila and Beijing over the South China Sea spiked following the detection of hundreds of Chinese boats parked at a reef off the Philippines.

On the eve of the joint drills, the Philippine Coast Guard accused its Chinese counterpart of steering one of its ships within metres of a Filipino patrol boat near the disputed Scarborough Shoal -- a flashpoint between the two countries.

That came weeks after Manila confronted Beijing's ambassador over a Chinese navy ship "lingering" in the Philippines' archipelagic waters.

The Chinese embassy in Manila did not immediately respond to media requests for comment on the war games or the Scarborough Shoal incident.

The exercises are being held in the shadow of the Russian invasion of Ukraine. The US and its allies are providing defensive weapons to Kyiv and imposing crippling economic sanctions on Moscow.

Duterte, whose six-year term ends in June, has expressed concern that the Philippines was "involved" in the conflict because of its security alliance with the United States.

That includes a mutual defence treaty and permission for the US military to store defence equipment and supplies on several Philippine bases.