GENEVA Syrian women demanded on Thursday that the fate of their detained or missing relatives be addressed in United Nations- sponsored peace talks, hoping they lead to prisoner releases.

Staffan de Mistura, U.N. Special Envoy for Syria, met them after seeing delegations from the Syrian government and opposition separately at the start of the first negotiations in nearly a year.

"They were sending to me, and through me to everyone, the message that while we are going to talk about the future of Syria we should not forget those who are still either detained or abducted, or missing," de Mistura told reporters.

"There are thousands and thousands of mothers, wives, daughters who are hoping that at least this aspect will be one of the benefits of any negotiation...You can be sure that we will be constantly raising detainees, abducted and missing people."

The women then read out a joint statement at a vigil, saying: "We, as families, demand the immediate release of our relatives who have been unlawfully detained."

Framed photos of dozens of prisoners - strewn with long-stemmed red roses - were placed on the ground.

"I am married, but to a detainee. He was arrested two weeks before our wedding on March 15, 2012," Noura Ghazi told Reuters. "Ten months later he was transferred to Adra prison in Damascus. We got married in prison."

She visited her husband, Bassel Khartabil, every week for nearly three years before he was transferred to an "unknown place" in October 2015, she said.

"There was a rumor he was sentenced to death. I know nothing," said Ghazi, a human rights lawyer who described her husband as a "peaceful activist".

Fadwa Mahmoud, a 63-year-old dissident, sought refuge in Berlin in 2015 three years after searching for her husband Abdulaziz Al Khayer, head of the banned Communist Action Party, and her son Maher Tahan, both arrested near Damascus airport.

"I cry every day," she said.

"I want to keep working on this file as long as there is one detainee in Syria. It is not only about my son and husband," added Mahmoud who was imprisoned under former President Hafez al-Assad, whose son Bashar has ruled Syria for 17 years.

Fadel Abdul Ghany, chairman of the Syrian Network for Human Rights, said 92,000 people are believed to be in the custody of authorities, including 4,000 women and 2,500 children.

"This is a regime strategy in order to destroy society," he told Reuters at the vigil.

Islamic State forces hold about 8,000 prisoners, while rebel groups fighting under the name of the Free Syrian Army have 2,400, the group formerly know as the Nusra Front about 500 and PYD Kurds about 1,600, he said.

"We have to find a way to deal with these important cases in order to achieve something in political negotiations."

(Editing by Dominic Evans)