The United States called on China on Saturday to stop "bullying behavior" in the South China Sea and said it was concerned by reports of Beijing's "provocative actions" aimed at offshore oil and gas developments in the disputed waters.
Three regional security sources told Reuters on Friday that a Chinese government survey ship was tagging an exploration vessel operated by Malaysia's state oil company Petronas in those waters.
The vessel Haiyang Dizhi 8 was earlier this week spotted off Vietnam, where it had last year conducted suspected oil exploration surveys in large expanses of Vietnam's exclusive economic zone.
"The United States is concerned by reports of China's repeated provocative actions aimed at the offshore oil and gas development of other claimant states," the U.S. State Department said in an emailed statement in response to questions on the Haiyang Dizhi 8's presence in Malaysian waters.
"In this instance, (China) should cease its bullying behavior and refrain from engaging in this type of provocative and destabilizing activity," it said.
China's actions threaten regional energy security and undermine the free and open Indo-Pacific energy market, it added.
Earlier in the week, when the survey ship was off Vietnam, a Chinese foreign ministry spokesman had said the ship was conducting normal activities and accused U.S. officials of smearing Beijing.
China claims almost all of the resource-rich South China Sea, also a major trade route. The Philippines, Brunei, Vietnam, Malaysia and Taiwan have overlapping claims.
The Haiyang Dizhi 8 was still within Malaysia's exclusive economic zone, Marine Traffic data showed on Saturday. The area is close to waters claimed by both Vietnam and Malaysia as well as by China.
A Malaysian security source had said the Haiyang Dizhi 8 was flanked at one point on Friday by more than 10 Chinese vessels, including those belonging to maritime militia and the coast guard. A Vietnamese vessel was also tagging the Petronas vessel.
(Reporting by A. Ananthalakshmi, Rozanna Latiff; Editing by Andrew Heavens and Ros Russell)
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