The Red Cross said on Friday it had opened two new telephone hotlines to try to reunite Syrians who have been missing for years with their families, but warned that many cases will take months or years to resolve.

Since the start of Syria's civil war over 13 years ago, the International Committee of the Red Cross (ICRC) has received over 35,000 cases of missing people and is stepping up its efforts to help trace them.

Stephan Sakalian, head of delegation for ICRC in Syria, told reporters that it had opened two hotlines this week: one for prisoners and one for families to try to connect them.

"We can provide them with mental health and psychosocial support ... we can even help them financially if they need to be reunited," he told a Geneva press briefing via video link from Damascus. Legal aid and healthcare are also available, an ICRC statement said. 

The opening of president Bashar al-Assad's detention system has raised hopes for reunions, with some prisoners re-emerging who were thought by their families to have been executed years ago. But Sakalian sought to temper expectations.

"Let's make no mistake: giving answers to people will take weeks, months and maybe years, given the amount of information to process," he said. "The work is tremendous," he added.

The ICRC is also looking for three of its colleagues who were abducted in 2013. "Like everyone we want to have hope and seek any signal or any news that may bring some closure to their families, but for the moment, we do not have any news," he added.