China must release detained rights lawyers and activists "immediately and without conditions", UN rights chief Zeid Ra'ad Al Hussein said in a statement Tuesday.

In a nationwide crackdown that began in July, China has detained about 250 "human rights lawyers, legal assistants, and activists", the statement said, adding that some have since been released.

According to Zeid's spokesman Rupert Colville, the UN rights chief has held talks with Chinese officials over the arrests, most recently at the weekend.

Zaid has raised concern over the "harassment and intimidation of government critics" and civil society workers, the statement said.

It specifically highlighted the January arrests of 15 human rights lawyers, 10 of whom faced charges of "subversion of state power".

Among those arrested last month were Li Heping and Wang Yu, two prominent lawyers.

"Lawyers should never have to suffer prosecution or any other kind of sanctions or intimidation for discharging their professional duties," Zeid said.

"I urge the Government of China to release all of them immediately and without conditions."

Officials from Beijing "too often reflexively confuse the legitimate role of lawyers and activists with threats to public order and security," Zeid said.

The statement also voiced concern over the recent disappearance of five employees of the Hong Kong publishing house Mighty Current, which continues to release works critical of the Chinese government.

After months of silence, Beijing acknowledged that these individuals were under criminal investigation.

Zeid called for "fair and transparent" procedures in the cases.

He also sounded an alarm over the early January arrest in Beijing of Swedish national Peter Dahlin, who co-founded a civil society organisation offering legal assistance called Chinese Urgent Action Working Group.

Dahlin was the first foreign national held on charges of "endangering state security" in China. He was expelled from the country on January 26.

Zeid meanwhile expressed regret over a growing trend of governments using national security to justify crackdowns on fundamental rights, which he said was happening increasingly "around the world."