According to Reuters, an exploration vessel owned by Malaysia's state oil company Petronas in those waters was identified by a Chinese government survey ship.

Earlier this week, Chinese-flagged Haiyang Dizhi 8 was spotted off Vietnam where it had conducted suspected oil exploration surveys in wide expanses of the exclusive economic zone of Vietnam last year.

"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 stated in an emailed statement.

Actions by China are threatening regional energy security and undermining the free and open Indo-Pacific energy market, she added.

Earlier in the week, while the survey ship was off Vietnam, a spokesperson for the Chinese foreign ministry said the ship was carrying out routine activities and accused U.S. officials of smearing Beijing.

China is claiming almost all of the resource-rich South China Sea, which is also an important trade route. There are conflicting statements on the Philippines, Brunei, Vietnam, Malaysia and Taiwan.

The Haiyang Dizhi 8 was still inside the exclusive economic zone of Malaysia, Marine Traffic data on Saturday revealed. The region is similar to the waters both claimed by Vietnam and Malaysia, and by China.

Maritime Business World