Syrian officials will be held accountable for the atrocities they committed against their own people, a senior group of European diplomats pledged.
With more than 150,000 Syrian men, women and children missing, many believed to have been tortured and killed, there are growing calls for justice in the decade-long conflict.
Accounts of husbands and children who disappeared were heard in an online seminar attended by ambassadors from Germany, France and Britain before the Brussels V Conference on Syria, on March 29-30.
"More than 150,000 Syrians have disappeared," said the German special representative for Syria, Robert Rohde.
"Fathers, mothers, sisters, brothers and friends have to ask themselves unbearable questions every day. 'Where is my husband? Is my daughter still alive?'
"People have been deprived of the most basic needs but, above all, their human dignity.
"More than 14,000 Syrians died under torture. The people of Syria paid the highest price."
Mr Rohde said Germany and its partners would hold the offenders accountable.
He referred to the recent conviction of a Syrian secret policeman who was jailed for four years last month in a Koblenz court for crimes against humanity.
It was first time a court outside Syria ruled on state-sponsored torture by the regime of Syrian President Bashar Al Assad and, Mr Rohde said, it meant war criminals "cannot feel safe anywhere".
"Koblenz was a historic landmark as it sends a clear message: whoever commits crimes against humanity will eventually be held accountable," he said.
The German government also supports groups gathering evidence on crimes, Mr Rohde said.
It was crucial that the voices of missing people's families be heard, said French representative Francois Senemaud.
Mr Senemaud said he listened to Syrians testify about "the most horrible crimes" being committed in prisons, and that the Assad regime had "shown no pity and refused any releases, even of women and children".
"After 10 years of impunity for the perpetrators of crimes committed in Syria, it has to stop," he said.
"We are proud of the co-operation between the German and French judiciary, in order to bring perpetrators to justice."
Mr Senemaud said the French judicial authorities were conducting 40 proceedings related to crimes committed in Iraq and Syria, and have issued arrest warrants.
Britain's special representative for Syria, Jonathan Hargreaves, said: "Unimaginable suffering has been endured by detainees."
Mr Hargreaves said it included "sexual violence, torture and executions", with thousands of families not knowing the fate of their loved ones.
"Imagine what it's like for so many Syrians, including some of those with us today, desperate for information," he said. "This really is a national trauma."
Mr Hargreaves said Britain would work with Syrian activists to build a central record of all those who are missing.
The conference, titled Arbitrary Detainees and the Forcibly Disappeared, was shown videos of families before they were broken apart.
"I am one of over 150,000 Syrians who are waiting to hear about our loved ones who were detained and disappeared," Nivin Almousa said.
"Nobody can bear the pain of a family that has been told, 'forget about him, he’s gone'."
Ms Almousa's brother Hamza, 33, went missing as he travelled home from university in Aleppo to Hama. His photo was among 28,000 smuggled out of Syria of those in detention.
"All groups that are involved in crimes against humanity in Syria must be stopped and must be held accountable," she said.
"I hope one day to see [Mr Al Assad] himself in front of an international tribunal or the International Criminal Court."
Noura Ghazi, 39, lawyer and activist for Syrian detainees, lost her husband Bassel when he was secretly executed in 2015 after three years in jail. She still has no idea where his remains are.
"I try to find a closure to this open wound in my heart that is bleeding, every single moment," Ms Ghazi said.
"I just want a grave to cry on, to put his favourite flowers on and to talk to him."
