About 5000 soldiers from the United States, Indonesia, Australia and four other nations have begun annual training exercises on the Indonesian island of Java as China's increasing aggression raises concern.
American and Indonesian soldiers have held the live-fire drill since 2009, and Australia, Japan and Singapore joined last year.
The UK and French forces are participating in this year's Super Garuda Shield exercises, with some 5000 personnel in total.
China sees the expanded drills as a threat, accusing the US of building an Indo-Pacific alliance similar to NATO to limit China's growing military and diplomatic influence in the region.
Brunei, Brazil, Canada, Germany, India, Malaysia, Netherlands, New Zealand, Papua New Guinea, Philippines, South Korea and East Timor also sent observers to the two-week exercises in Baluran, a coastal town in East Java province.
The commanding general of US Army Pacific, General Charles Flynn, said the 19 nations involved in the training were a powerful demonstration of multilateral solidarity to safeguard a free and open Indo-Pacific region.
"Super Garuda Shield 2023 builds on last year's tremendous success," Flynn said in a statement released by the US Embassy in Jakarta on Tuesday.
"This joint, multinational training exercise displays our collective commitment and like-minded unity, allowing for a stable, secure, and more peaceful, free and open Indo-Pacific."
The statement said at least 2100 US and 1900 Indonesian forces would enhance interoperability capabilities through training and cultural exchanges including a command and control simulation, an amphibious exercise, airborne operations, an airfield seizure exercise, and a combined joint field training with a live-fire event.
The command post exercise will focus on mission planning staff tasks in a combined military setting.
A field training exercise will involve battalion-strength elements from each nation exercising war-fighting skills to enhance interoperability and combined operational capacity.
Garuda Shield was being held in several places, including in waters around Natuna at the southern portion of the South China Sea, a fault line in the rivalry between the US and China.
Indonesia and China enjoy generally positive ties, but Jakarta has expressed concern about what it sees as Chinese encroachment in its exclusive economic zone in the South China Sea.
The edge of the exclusive economic zone overlaps with Beijing's unilaterally declared "nine-dash line" demarking its claims in the South China Sea.
Increased activities by Chinese coast guard vessels and fishing boats in the area have unnerved Jakarta, prompting Indonesia's navy to conduct a large drill in July 2020 in waters around Natuna.