Rights group Amnesty International said Wednesday that it fears for the health of prominent Syrian human rights lawyer Khalil Maatuq and his colleague, a year after they disappeared.
Maatuq and Mohamed Thatha were "reportedly arrested while on their way to their office in Damascus" a year ago, said the London-based group, although the government has denied holding them.
"There are growing fears for (Maatuq's) health. (He) suffers from advanced lung disease and has severe breathing difficulties," said Amnesty.
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"As director of the Syrian Centre for Legal Studies and Research, (Maatuq) was a thorn in the side of the Syrian government and, as a human rights lawyer, he has defended hundreds of political prisoners and prisoners of conscience over many years," it added.
The group said it had designated Maatuq a "prisoner of conscience, detained solely for his peaceful activities," adding that it believes the two men "have been subjected to enforced disappearance."
Calling for them to be freed "immediately and unconditionally," Amnesty's Middle East and North Africa programme director Philip Luther said enforced disappearance is "a crime under international law that puts both men at increased risk of torture... and extrajudicial execution."
Activists say tens of thousands of people are being held by the Syrian authorities, many of them arrested arbitrarily since the outbreak of the revolt against President Bashar al-Assad in March 2011.
For years, human rights groups have documented the systematic use of torture in Syria's jails.