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 Haiyang Dizhi 8 was still within Malaysia's exclusive economic zone, Marine Traffic data showed on Saturday.

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.

Vietnam calls the South China Sea the East Sea.

Vietnam Foreign Ministry spokeswoman Le Thi Thu Hang, upon questioned of the Chinese survey ship's presence in Vietnam's EEZ, said on April 14 that Vietnam always monitors East Sea developments and insists that all nations comply with international laws.

"Vietnam demands that all nations follow regulations of the 1982 United Nations Convention on the Law of the Sea (UNCLOS) and international rules in all activities, contributing to building peace, stability and cooperation in the East Sea." Hang said.

In July last year, the Haiyang Dizhi 8 and its escort vessels had violated Vietnam’s EEZ and the continental shelf of Vietnam near the Vanguard Bank, occupying the area until August 7. The vessels returned to the water area on August 13 and stayed until October.

Vietnam repeatedly condemned the operations of the vessel and escorts as a violation of its sovereignty and demanded that they leave Vietnamese waters immediately.