Christian lawyer Gao Zhisheng will have been missing for 150 days on Saturday. The Nobel Peace Prize nominee was last seen being hauled away by Chinese officials on February 4, according to Christian Solidarity Worldwide (CSW).

The persecution watchdog said it was “gravely concerned” for the lawyer, who has been repeatedly arrested, tortured and imprisoned since 2005 as a result of his work defending other human rights activists and religious minorities, particularly house church leaders and Falun Gong followers.

He was arrested in 2007 after sending an open letter to the US Congress detailing human rights abuses in the lead up to the Olympic Games. He later wrote another open letter in which he claimed to have been tortured by Chinese authorities for more than 50 days as a result of his first letter.

Gao said he was threatened with death if he spoke openly about the torture and his disappearance is believed to be connected to his torture claims. China Aid Association says its sources inside China are concerned that Gao is currently being tortured.

International pressure led the Chinese Ambassador to the US, Zhou Wenzhong, to issue a statement in which he said Gao was “currently serving probation” but did not offer any information on Gao’s whereabouts.

“The public security bureau has taken no mandatory action against him,” he insisted.

According to CSW, Gao once predicted that every human rights lawyer in China would one day become a human rights case. His wife and two children, who have fled to the US out of fears for their safety, are particularly concerned for his welfare.

CSW’s Advocacy Director, Tina Lambert, called for Gao’s immediate release.

“His case symbolises the continuing threat faced by human rights defenders in China and is a sign of the wider human rights violations, which continue to be perpetrated by State actors,” she said.

“We urge China to match its economic liberalism with serious improvements in human rights.”