"Recently China resumed its coercive interference in Vietnam's longstanding oil and gas activities in the South China Sea," it said in a statement on Monday.

The South China Sea is known as the East Sea in Vietnam.

The Pentagon said Beijing’s activities were contradictory to Chinese Defense Minister Wei Fenghe’s pledge in a speech in Singapore earlier this year that China would "stick to the path of peaceful development."

"China will not win the trust of its neighbors nor the respect of the international community by maintaining its bullying tactics.

"Its actions to coerce ASEAN claimants, station offensive military systems, and enforce an unlawful maritime claim raise serious doubts over China's credibility."

The Pentagon said Washington would continue its efforts to ensure freedom of navigation and economic opportunity throughout the Indo-Pacific.

The statement came in the context of Chinese oil survey vessel Haiyang Dizhi 8 and escort vessels illegally reentering Vietnamese waters near the Vanguard Bank in the southern part of the South China Sea on August 13.

The vessels had left Vietnam’s exclusive economic zone (EEZ) and continental shelf on August 7 after trespassing for a month.

Vietnamese Foreign Ministry spokeswoman Le Thi Thu Hang said at a press briefing last Thursday that Vietnam had on several occasions called on China to withdraws its ships.

The U.S. State Department said last week it was deeply concerned about China's interference in oil and gas activities in waters claimed by Vietnam, and the deployment of the vessels was "an escalation by Beijing in its efforts to intimidate other claimants out of developing resources in the South China Sea."

In a tweet on August 20 U.S. National Security Advisor John Bolton accused China of "bullying."

The issue of Chinese vessels intruding into Vietnamese waters in the South China Sea had cast a shadow over the 52nd ASEAN Foreign Ministers' Meeting held in Bangkok, Thailand, between July 29 and August 3.

Vietnam’s Deputy Prime Minister and Minister of Foreign Affairs Pham Binh Minh had denounced China's activities at the meeting, calling them "illegal" and "serious violations of Vietnam's sovereignty and jurisdiction."

His counterparts from the U.S., Japan and Australia also expressed concern over "aggression" and interference with oil and gas activities in the South China Sea.

China claims almost the entire South China Sea, including waters close to Taiwan and Southeast Asian countries Brunei, Malaysia, the Philippines, and Vietnam.