The first Asean-US Maritime Exercise (AUMX) between the regional bloc and Washington lasts for five days, starting at the Sattahip Naval Base in Thailand and ending in Singapore.

The drills come at a time of stepped-up US engagement in the region and tensions between Beijing and Southeast Asian nations over the South China Sea, parts of which are claimed by Brunei, Malaysia, Vietnam and the Philippines.

Co-led by the US and Royal Thai navies, the exercises will stretch into "international waters in Southeast Asia, including the Gulf of Thailand and South China Sea" before concluding in Singapore, according to a statement from the US embassy in Bangkok.

"AUMX builds greater maritime security on the strength of ASEAN, the strength of our navy-to-navy bonds, and the strength of our shared belief in a free and open Indo-Pacific," said Rear Admiral Joey Tynch, who oversees the US Navy's security cooperation in Southeast Asia.

The joint drills have come under criticism for looping in Myanmar's Navy in a rare show of inclusion despite Washington imposing sanctions on top army brass over the Rohingya crisis.

All ten members of the Association of Southeast Asian Nations will take part in the exercises which include the boarding of target vessels to simulate search and seizure.

They are unfolding as a Chinese survey ship remains in waters claimed by Vietnam, prompting the Pentagon last week to accuse Beijing of efforts to "violate the rules-based international order throughout the Indo-Pacific."

China claims the majority of the South China Sea, often invoking its so-called nine-dash line as a supposed historic justification to the waters, which are a key global shipping route.

On a trip to Thailand last month, US Secretary of State Mike Pompeo urged Southeast Asian nations to push back against Chinese "coercion" in the sea.