Rights groups and activists are sounding the alarm about the detention of two Chinese investigative journalists after they reportedly exposed corruption by a senior official in Sichuan province.

Independent journalists Liu Hu and Wu Yingjiao were taken by police on Sunday after they published their investigation, rights groups say.

Critics have long flagged concern about media repression in China, where authorities have arrested and prosecuted journalists, accusing them of causing trouble.

Chengdu police said on Monday that two men, identified by their surnames Liu and Wu and aged 50 and 34, were under investigation for "making false accusations" and "illegal business operations".

In response a question about the arrests, a Chinese foreign ministry spokesperson told reporters on Wednesday that relevant authorities had already released information on the case.

"China is a country ruled by law, and Chinese judicial organs handle cases according to law; everyone is equal in face of the law," the spokesperson said.

Liu, a prominent investigative reporter in China, was previously arrested in 2013 on suspicion of defamation after accusing a high-ranking official of corruption.

After his release in 2014, he continued his investigations and published his reports on social media.

Wu is frequently listed as Liu's collaborator on a public WeChat account where Liu and other journalists post news articles.

According to Chinese Human Rights Defenders, which is a coalition of Chinese and international activists and groups, Liu had planned to take a train from his home city of Chongqing to the capital Beijing on Sunday when he went missing.

Wu was taken by police in Hebei province on the same day, the group said.

According to rights groups, the pair's recent report had exposed a county official's alleged corruption.

The title of the report - which cannot be found on WeChat any more - had referred to businesses driven to bankruptcy due to the official's actions.

Before Liu's arrest this week, he had posted to WeChat several messages he had received from a Chengdu disciplinary inspection official urging him to contact authorities rather than publish reports in the media, Reporters Without Borders (RSF) said.

"This arrest highlights just how restrictive and hostile China has become toward independent reporting," RSF advocacy manager Aleksandra Bielakowska said in a statement.

"We call on the international community to intensify pressure on the Chinese regime, rather than pursue a normalisation of relations that only enables further repression and allows the authorities to continue targeting reliable reporters."

There are more than 120 journalists detained in China, according to RSF, which calls the country "the world's biggest jailer of journalists".