UN-Arab League peace envoy Lakhdar Brahimi on Thursday demanded the release of leading Syrian women's rights campaigner Razan Zeitouneh and three fellow activists, abducted last week by unknown kidnappers.
"We have all got to demand that they be released," Brahimi told activists from Syria and other countries as well as diplomats at a meeting at the UN's Geneva offices.
"Zeitouneh should have been here today, and I hope that she will be here on the 12th of January," he added, referring to a gathering of Syrian women campaigners planned ahead of long-awaited peace talks between the government and rebels starting in Switzerland on January 22.
Zeitouneh was among the 2011 winners of the European Parliament's top human rights prize for their role in the Arab Spring.
She, her husband Wael Hamada, and fellow campaigners Samira Khalil and Nazem al-Hamadi were seized on December 10 by unknown attackers who raided the offices of a group documenting human rights abuses.
The Local Coordination Committees (LCC) activist network said it believed rebels were responsible for the kidnapping in Damascus province, not far from the capital.
Multiple rebel groups, including the jihadist Al-Nusra Front and Islamic State of Iraq and the Levant, operate in the area.
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Zeitouneh, one of the Syrian uprising's most prominent early supporters, has criticised abuses by all sides in the conflict.
The LCC backs the uprising against Syrian President Bashar al-Assad but opposes the militarisation of the conflict.
It documents deaths in the civil war, which was ignited by a March 2011 crackdown on pro-democracy protests and is now estimated to have claimed some 126,000 lives.
Brahimi also spotlighted the case of Raja al-Nasser, a high-profile member of a regime-tolerated opposition party arrested last month in Damascus allegedly by government agents.
Nasser was a likely delegate for next month's peace talks.
"We were told first that he was arrested by mistake. Second we were told that he would be released soon. Third we were told that he has been released. But we see that he has not been released. And now we are told, 'We don't know where he is'," Brahimi protested.
His remarks came as a UN-mandated inquiry team accused government forces of waging a systematic campaign of enforced disappearances to terrorise the population, amounting to a crime against humanity.
The probe also said that kidnappings by rebels could constitute a war crime.