"China’s recent escalation of efforts to intimidate others out of developing resources in the South China Sea is disturbing," the U.S. National Security Advisor John Bolton tweeted.

"The United States stands firmly with those who oppose coercive behavior and bullying tactics which threaten regional peace and security."

Bolton's statement came as Chinese oil survey vessel Haiyang Dizhi 8 and escorts returned to Vietnam’s exclusive economic zone (EEZ) and continental shelf on August 13, less than a week after leaving on August 7.

The Chinese vessels had infringed on Vietnamese sea territory, around the Vanguard Bank in the southern area of the South China Sea, which Vietnam calls the East Sea, for a month in their first incursion.

Vietnam’s Foreign Ministry spokeswoman Le Thi Thu Hang said on August 16 China’s repeated infringement of Vietnamese waters is counterproductive to regional peace and stability.

She said Vietnam has conveyed to China its opposition to such serious violations being repeated and requested that China withdraw all the vessels from Vietnamese waters.

General Charles Brown Jr., Commander of Pacific Air Forces and Executive Director of the Pacific Air Combat Operations, said during a press conference in Hanoi on Sunday that Beijing’s actions in the South China Sea go against the U.S.’s goal of maintaining an open and free Indo-Pacific region.

In some cases, China’s actions affect the sovereignty and the EEZs of several countries in the waters, he added.

Secretary of State Mike Pompeo spoke out against Chinese "coercion" of Southeast Asian neighbors in disputes over the South China Sea at the ASEAN Ministers' Meeting in Bangkok early this month, which was also attended by his Chinese counterpart Wang Yi.

Before the meeting, U.S. Senators had urged him to accord high priority to Chinese maritime aggression, calling for a regional consensus with the U.S.’s allies and partners to end Chinese infringements in the South China Sea.