China's Haiyang Dizhi 8 entered waters near Malaysia on Thursday, according to ship tracking website Marine Traffic.
On Friday, it was close to the Petronas-operated West Capella, according to the security sources, who did not want to be identified because they were not authorized to talk to the media. One of the sources said a Vietnamese vessel was also tagging the West Capella.
The area is close to waters claimed by both Vietnam and Malaysia as well by China, through its sweeping claim to most of the South China Sea within its U-shaped 'nine-dash line' that is not recognized by its neighbors or most of the world.
The United States has accused China of taking advantage of the distraction of the coronavirus pandemic to advance its presence in the South China Sea.
On Wednesday, a Chinese foreign ministry spokesman said the Haiyang Dizhi 8 was conducting normal activities and accused U.S. officials of smearing Beijing.
A Malaysian security source 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. That source also mentioned the Vietnamese vessel.
The Malaysian prime minister's office, the defense ministry and Petronas did not respond to requests for comment.
Zubil Mat Som, the head of Malaysia's maritime enforcement agency, confirmed that the Haiyang Dizhi 8 was in Malaysian waters, Malay-language daily Harian Metro reported.
"We do not know its purpose but it is not carrying out any activities against the law," he said, according to the report.
The foreign ministries of China and Vietnam did not immediately respond to requests for comment.
The Haiyang Dizhi 8 was 324 km (200 miles) off the Malaysian coast, within Malaysia's exclusive economic zone, Marine Traffic data showed.
It was flanked by a Chinese coastguard vessel as it moved in a hash-shaped pattern consistent with carrying out a survey, as it did during a tense standoff in Vietnamese waters last year.
The West Capella was not visible on the tracking data, but the security sources said the vessel was in the area. The Asia Maritime Transparency Initiative, a Washington DC-based think tank, has signaled its presence there since October.
China has sent medical help to Malaysia, which has reported more than 5,000 coronavirus infections.
China claims almost all of the energy-rich South China Sea, also a major trade route each year. The Philippines, Brunei, Vietnam, Malaysia and Taiwan have overlapping claims.