In a press statement released Thursday, State Department spokeswoman Morgan Ortagus said: "This calls into serious question China’s commitment, including in the ASEAN-China Declaration on the Conduct of Parties in the South China Sea, to the peaceful resolution of maritime disputes."

She said the redeployment of oil survey vessel Haiyang Dizhi 8 in Vietnamese waters was "an escalation by Beijing in its efforts to intimidate other claimants out of developing resources in the South China Sea."

The survey vessel and its escorts had left the nation’s exclusive economic zone (EEZ) and continental shelf on August 7 after occupying Vietnamese sea territory for about a month. However, it returned to Vietnamese waters near the Vanguard Bank in the southern area of the South China Sea on August 13.

Vietnamese Foreign Ministry spokeswoman Le Thi Thu Hang said at a press meet Thursday that Vietnam has made contact multiple times and requested that China withdraws its ships.

Ortagus noted that "China has taken a series of aggressive steps to interfere with ASEAN claimants’ longstanding, well-established economic activities, in an attempt both to coerce them to reject partnerships with foreign oil and gas firms, and to work only with China’s state-owned enterprises.

"In the case of Vanguard Bank, China is pressuring Vietnam over its work with a Russian energy firm and other international partners," she added.

China's actions also demonstrate its disregard for the rights of countries to undertake economic activities in their EEZs under the 1982 Law of the Sea Convention, Ortagus said.

In July, after China sent the survey ship and escorts to Vietnamese waters the first time, the State Department also voiced its concerns.

"China’s repeated provocative actions aimed at the offshore oil and gas development of other claimant states threatens regional energy security and undermines the free and open Indo-Pacific energy market," it said in the previous statement.

It cited Secretary of State Mike Pompeo's comments earlier this year when he said that "by blocking development in the South China Sea through coercive means, China prevents ASEAN members from accessing more than $2.5 trillion in recoverable energy reserves."

The issue of Chinese vessels infringing upon Vietnamese waters in the South China Sea, which Vietnam calls the East 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 ASEAN meeting, calling them illegal and serious violations of Vietnam's sovereignty and jurisdiction rights.

His counterparts from the U.S., Japan and Australia also expressed concerns at the meeting over aggression that intimidate oil and gas activities in the South China Sea.

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

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