“Vietnam should be happy” that the drills are taking place given China’s recent “aggression in its waters,” said Luc Anh Tuan, a researcher at the University of New South Wales in Australia.

“Hanoi nevertheless will manage to downplay the significance of the drill because like other ASEAN fellows, it does not want to create an impression of a coalition against China,” added Mr. Tuan, who is on educational leave from the Vietnamese Ministry of Public Security.

The Vietnamese Foreign Ministry confirmed in an email last week that the drills were happening, but declined to answer other questions.

Beijing’s actions in the sea are hugely sensitive for Hanoi because it is under heavy domestic pressure to be tough on China, its largest trading partner and former colonial occupier. But Vietnam is also racing to find new energy sources to power its fast-growing economy.

In a sign of those tensions, there were rare anti-Chinese riots in Vietnam in 2014, after a state-owned Chinese company defiantly towed an oil rig into disputed waters near the Vietnamese coast, prompting a tense maritime standoff. Three years later, Vietnam suspended a gas-drilling project in the sea by a subsidiary of a Spanish company because the project was said to have irritated Beijing.

Yan Yan, an expert on maritime law at the National Institute for South China Sea Studies, on Hainan Island off the Chinese mainland, said that this week’s drills reflected Washington’s “concern and anxiety” about waning American influence in the region. She said the drills were not a cause for concern for Beijing, which she said planned to eventually expand the “subjects and scope” of its own naval drills with ASEAN.