Eight warships, three MH60 helicopters and a P-8 Poseidon aircraft and more than 1,000 soldiers from the US and the 10 Asean countries were participating in the exercises, which began at the Thai naval base of Sattahip, according to a statement by the US Department of Defence.
The drills will last till Saturday and will pass through the Gulf of Thailand and parts of the South China Sea before concluding in Singapore, Efe news reported.
The US Navy has already carried out joint exercises with some Southeast Asian countries, but this is the first time that the entire bloc is participating in joint drills with the US, although Asean carried out similar exercises with China last year, the report said.
Rear Adm Joey Tynch the Task Force 73 commander who oversees US Navy security operations in the region said the drills would boost maritime security in the area, improve ties between navies and strengthen their "shared belief in a free and open Indo-Pacific".
The military exercises come at a time of heightened tensions after Vietnam reported the presence of a Chinese oil survey ship along with armed escorts in July, in what Hanoi regards as its territorial waters.
Although Beijing claims that the area comes under its maritime sovereignty, Vietnamese authorities demanded the withdrawal of Chinese ships from the area on two occasions within the last month, an appeal which was also backed by the US.
Beijing claims the strategically important South China Sea - which provides passage to large volumes of international trade and is rich in natural resources - in its entirety, but its waters are also partially claimed by Brunei, the Philippines, Malaysia and Vietnam.
All four parties to the dispute apart from China are members of Asean, which also includes Thailand, Singapore, Cambodia, Laos, Myanmar and Indonesia.
( With inputs from IANS )