BEIRUT: Amnesty International called Thursday on the Syrian authorities either to charge or release two human rights activists detained in Damascus earlier in the week.

Lawyer Khalil al-Maatouq and his colleague Mohammed Thatha were arrested while on their way to work on Tuesday, according to their employer, the Syrian Centre for Legal Studies and Research.

"The families of these men have a right to know what has happened to them," said Ann Harrison, deputy director of Amnesty's Middle East and North Africa programme.

"If they are held, they should be released immediately and unconditionally unless they are charged with an internationally recognisable offence," she said in a statement.

Maatouq requires medical care for a serious lung condition, she added.

Human Rights Watch said Maatouq and his colleague were "abducted" while driving to work and have not been seen since, adding that his family suspect they are being held incommunicado.

"The Syrian government should immediately release him if he is in its custody," it said.

Thousands of people have been detained in Syria since the outbreak of a revolt against President Bashar al-Assad in March last year, many of them without charge, human rights groups say.

"Maatouq's disappearance is a stark reminder of the grave risks that brave human rights activists are taking in Syria just to document the daily horrors," said Sarah Leah Whitson, HRW's director for the Middle East and North Africa.