Malaysia will protest against what it called the intrusion of a Chinese Coast Guard ship into its waters north of Borneo, the Wall Street Journal reported, in another departure from the country's previously soft approach to South China Sea disputes.
National Security Minister Shahidan Kassim was quoted as telling the Journal that Malaysian Prime Minister Najib Razak would raise the issue directly with Chinese President Xi Jinping.
The report comes after Shahidan posted pictures on Facebook of what he said was a Chinese Coast Guard ship anchored at Luconia Shoals, an area of islets and reefs about 150 km (93 miles) north of Malaysian Borneo.
The shoals are inside the 200 nautical mile exclusive economic zone claimed by Malaysia and about 2,000 km from mainland China, he said, adding that any foreign vessels entering the area were "intruders".
"This is not an area with overlapping claims. In this case, we're taking diplomatic action," Shahidan was quoted as saying in the report on Monday.
Shahidan could not immediately be reached for comment.
Malaysia has generally adopted a cautious line in its dealings with Beijing over disputed territory in the South China Sea, in contrast to other claimants Vietnam and the Philippines, which have railed against perceived Chinese expansionism.
But two Chinese naval exercises in quick succession around the James Shoal, which lies inside Malaysia's exclusive economic zone, prompted Kuala Lumpur to change its approach last year, senior diplomats have previously told Reuters.
More recently, Beijing has angered its Southeast Asian neighbours and caused consternation in Washington and Tokyo by creating artificial islands in the potentially mineral-rich waters, with harbours and runways fit for military use.
Malaysia, the Philippines, Vietnam, Brunei and Taiwan claim parts of the sea, while China claims 90 percent of the 3.5 million sq km (1.35 million sq mile) area based on maps no other country recognises.
The United States has said that while it does not take sides in the territorial dispute, it is determined to keep vital shipping lanes open to international trade.