Now in its 42nd year, Cobra Gold brings thousands of forces from the United States, Thailand and other countries together for days of training.

Washington has sought to strengthen military alliances in the region in the face of an assertive China.

American, Thai and South Korean soldiers launched a joint amphibious operation near a navy base in the east of the country, in which jet fighters screamed overhead and tanks rumbled over the sand.

Personnel from Singapore, Indonesia, Malaysia, South Korea and Japan also took part in the exercises, which span air, land and sea, and feature a jungle survival course.

With around 6,000 members, the US delegation was more than four times the size of the contingent sent in 2022.

Last month, the United States and the Philippines agreed to resume joint patrols in the disputed South China Sea, and struck a deal to give US troops access to four more military bases in the Philippines.

Seoul and Washington have recently bolstered joint military drills following a year of sanctions-busting weapons tests by North Korea, infuriating Pyongyang, which sees such joint exercises as rehearsals for invasion.

"While we all speak different languages, we come together with common bonds in pursuit of a free and open Indo-Pacific, which is what Cobra Gold is all about," US Marines captain Jonathan Coronel told reporters.

"Thailand, the US and South Korea: we have a good cooperation in the region," said Cherngchai Chomcherngpat, commander in chief of the Royal Thai Navy.

"It is very important to train together... If there are tensions in the region, we know each other."