The United States on Thursday sharpened its criticism of China's activities in disputed areas of the South China Sea, slamming an "escalation" in efforts to intimidate other claimants" such as Vietnam.
China redeployed a government-owned survey vessel -- with armed escorts -- into the waters off Vietnam earlier this month, the US said.
Hanoi says those waters are part of its Exclusive Economic Zone (EEZ).
The move could reignite a spat between Beijing and Vietnam over rights to the resource-rich waters in the South China Sea. Other countries in Southeast Asia also have claimed parts of the sea.
"The United States is deeply concerned that China is continuing its interference with Vietnam's long-standing oil and gas activities in its EEZ," State Department spokeswoman Morgan Ortagus said.
In a statement, she called the deployment of the survey vessel "an escalation by Beijing in its efforts to intimidate other claimants out of developing resources in the South China Sea."
The US said these actions were intended to "coerce" the countries to "reject partnerships with foreign oil and gas firms, and to work only with China's state-owned enterprises."