UN human rights experts have urged China immediately to investigate the fate of a prominent Chinese rights lawyer missing for over two weeks, the latest example of a growing crackdown on legal activism.

Jiang Tianyong took on numerous high-profile rights cases, including those of Falun Gong practitioners, Tibetan protesters, victims of the 2008 contaminated milk powder scandal and those of fellow lawyers Gao Zhisheng and Chen Guangcheng before being disbarred for his activism in 2009, according to campaign group Amnesty International.

He disappeared on November 21 en route to Beijing from Changsha, the capital of the central province of Hunan, where he had gone to inquire about the situation of a detained human rights lawyer.

"We fear that Mr Jiang's disappearance may be directly linked to his advocacy and he may be at risk of torture," a group of three UN experts said in a statement Tuesday, adding that they could not "rule out the possibility" that state agents were to blame for his disappearance.

UN Special Rapporteur on extreme poverty and human rights Philip Alston met Jiang on a visit to China in August and said he feared the disappearance was in part a retaliation against the lawyer's assistance to UN experts.

"The international standards are clear: states must refrain from and protect all persons from acts of reprisal" for such cooperation, Alston said.

President Xi Jinping has overseen a tightening of controls on civil society since assuming power in 2012, closing avenues for legal activism that had opened up in recent years.

While the government initially targeted political activists and human rights campaigners, it has increasingly turned its attention to the lawyers who represent them.

In the most striking example, authorities detained more than 200 people last year during the so-called "709 crackdown" -- named after the July 9th date of the roundup -- including lawyers who took on civil rights cases considered sensitive by the ruling Communist Party.

Jiang had met the wife of Xie Yang, a lawyer detained in the crackdown, and visited the Changsha detention centre where Xie was held. He was due to board an overnight train back to Beijing following the meetings.

Authorities in Beijing, Changsha and Jiang's home town of Zhengzhou have reportedly refused to investigate his disappearance, according to the UN statement.

"Everyone in China needs to comply with the law, and whoever violates the law will be punished in accordance with our laws," said foreign ministry spokesman Lu Kang at a regular press briefing Wednesday in response to the UN statement.

Jiang was last detained in 2014 after attempting to investigate a "black jail" that allegedly held Falun Gong practitioners. He suffered a beating while detained that fractured eight of his ribs, according to Amnesty.