Move follows publication of leaked document warning Malaysia off oil drilling by Philippine media outlet.

Malaysia’s Ministry of Foreign Affairs has said it is investigating the leak of a classified document China sent to its embassy in Beijing in February following the publication last month of an article in a Philippine media outlet.

The ministry said police would be informed and an internal investigation was under way, in a statement released late on Wednesday.

It did not name the outlet or confirm the authenticity of the note’s contents but did go into detail about Malaysia’s claims in the disputed South China Sea.

The Philippine Daily Inquirer reported on August 29 that China had sent the Malaysian embassy a diplomatic note to stop all oil exploration and drilling activities at Luconia Shoals, which lie about 100 kilometres off the Malaysian state of Sarawak.

Quoting the note, the paper reported China claiming Malaysia’s activities “infringed” on its sovereignty.

The shoals are some 1,300km from Hainan Island, China’s nearest major landmass.

“The ministry views the leak of this document, which constitutes an official communication channel between the two countries, with grave concern,” the statement said.

Beijing claims almost the entire South China Sea under a nine-dash line that was ruled without merit in 2016 by an international court.

Malaysia, the Philippines, Vietnam and Taiwan also claim parts of the waterway.

Tensions have soared in recent months with repeated confrontations between Chinese and Philippine coastguard vessels at Second Thomas Shoal and other disputed areas.

Malaysia’s Foreign Ministry noted that Beijing and Kuala Lumpur had committed to resolving the South China Sea dispute peacefully.

“Malaysia’s stance on the South China Sea remains unchanged,” the statement said. “Malaysia will continue to defend its sovereignty, sovereign rights and interests in its maritime areas … This approach aligns with the universally recognised principles of international law, including the United Nations Convention on the Law of the Sea [UNCLOS], 1982.”

China’s embassy in Kuala Lumpur did not respond to phone and emailed requests for comment.