SHANGHAI - China's market regulator said on Friday it had launched an antitrust investigation into the country's largest online academic database, the China National Knowledge Infrastructure (CNKI).

The State Administration for Market Regulation published the announcement on its website. It did not provide further details.

CNKI, whose parent company is Tsinghua Tongfang, a Chinese state-owned software company linked to China's elite Tsinghua University, said in a statement on WeChat it would cooperate fully with the probe and it would take it as an opportunity to carry out a self-examination.

CNKI came under scrutiny last month after local media reported China's top research group, the Chinese Academy of Sciences, said it would suspend its use of CNKI, citing high subscription fees.

The South China Morning Post said that over the past decade at least six Chinese universities suspended the use of CNKI over its fee increases.

CNKI said on its website that at end-2017 it had the world's largest readership, with more than 20 million individual users and an average daily retrieval volume of over 10 million.

Asked about the controversy over CNKI fees, Tsinghua Tongfang told investors on Monday that both it and CNKI were aware of "public opinion" and that CNKI was looking to improve and make rectifications to its business model.

(Reporting by Brenda Goh Editing by Mark Potter)