The US and Thailand launched annual joint 'Cobra Gold' military exercises Tuesday as Asia grapples with uncertainty over the direction of American foreign policy under President Donald Trump.

The war games, typically involving days of gruelling and sometimes gruesome drills in the Thai jungle, are the largest US-led military exercises in Asia.

They have continued in spite of tensions over a 2014 coup in Thailand, prompting Washington to cut back on military aid and call for a return to democracy.

But the US has also been wary of pushing its long-standing Southeast Asian ally into the embrace of superpower rival China.

This year Washington sent the head of the US Pacific Command Admiral Harry Harris to attend the opening of Cobra Gold on Tuesday -- the highest ranking officer to visit Thailand since the coup.

He urged the country to resume democratic rule, a hand-off the junta has repeatedly delayed.

"We look forward to Thailand's reemergence as a flourishing democracy because we need Thailand to be a strong and stable partner," he said.

This year's 10-day exercise kicks off under a cloud of uncertainty about the role of the US in the region.

It is unclear whether Trump will rollback on his predecessor's much-trailed 'Asia pivot', which banked on boosting security and trade ties as a counterweight to China's growing might.

Trump's administration is also expected to take a softer line on human rights, potentially offering some respite for the Thai junta and other authoritarian governments in the region.

Some 3,600 US personnel will descend on Thailand's Chonburi and Nakhon Ratchasima provinces for the war games, along with soldiers from Singapore, Japan, South Korea, Indonesia and Malaysia.

The drills, which have been hosted by the US and Thailand since 1980, have previously involved an array of arduous activities, including a jungle survival challenge requiring US marines to drink snake blood.

US embassy spokeswoman Melissa Sweeney told AFP this year's drills will focus on humanitarian assistance and disaster relief.

China and India will join for those humanitarian exercises, with more than a dozen other countries sending representatives to observe.