Human rights groups have called for the release of Vietnamese Christian lawyer Nguyen Van Dai and his colleague, Le Thu Ha.

The lawyers were arrested in Hanoi on 16 December and accused of propaganda against the state, punishable by up to 20 years in jail.

“We appeal to the Vietnamese government to honour its international and domestic obligations and to release Mr. Nguyen and Ms. Le immediately and unconditionally,” said the statement signed on 6 Jan. by 26 human rights groups based in Indonesia, Thailand, Vietnam, the United States and Europe.

Dai had already served four years in prison and another four years of house arrest. He was also earlier attacked by masked assailants on 6 Dec. Fellow activists said the attack was a result of his work teaching students about free society and the rule of law. In March 2015, Dai said he suffered regular intimidation by Vietnamese security officers.

One local source told UCAN the arrests were “security measures the government takes to silence dissidents and human rights activists”, ahead of the Communist Party congress in Hanoi from 20-28 Jan.