Chinese citizen journalist Zhang Zhan has been released from prison after serving four years for her coverage of Beijing's Covid-19 response, according to a video released by media watchdog Reporters Without Borders.

However, rights groups said Zhang was still under surveillance and that her freedom was extremely limited.

Zhang, a former lawyer, travelled to the central city of Wuhan in February 2020 to report on the chaos at the pandemic's epicentre, questioning the authorities' handling of the outbreak in her smartphone videos.

She was detained in May 2020 and sentenced seven months later to four years in jail for "picking quarrels and provoking trouble" -- a charge routinely used to suppress dissent.

Zhang was due to be released on May 13 but a lack of information about her whereabouts had sparked concern among rights groups and activists that she could still be detained.

A short video featuring a pyjama-clad Zhang shared by Reporters Without Borders (RSF) on Wednesday appeared to show she had been released as scheduled.

"Police released me from prison at five in the morning on May 13 and sent me to my older brother's home in Shanghai," Zhang said in the video, in a soft, halting voice.

"Thank you everyone for your help and concern, I wish you all the best... there is not much more that I can say," Zhang said.

It was unclear who filmed the video, with RSF saying it was released "via an intermediary".

While Zhang appears to have been released, "her contact with the outside world and daily life are all under surveillance", Jane Wang, a UK-based activist involved in the campaign to release and locate Zhang, wrote on social media platform X.

"She only has limited freedom," Wang said.

Activists as well as the United Nations' rights office have expressed concern about 40-year-old Zhang's health after she conducted several hunger strikes in prison to protest against her conviction.

"RSF remains concerned by her situation and emphasises that partial freedom is not freedom at all," the media watchdog wrote in a statement on Wednesday.

"Diplomatic intervention remains crucial to ensure her full and unconditional release without delay," RSF said.