The UN rights chief on Thursday urged Syria's warring parties to disclose the location of tens of thousands of missing people, as she marked 10 grim years since the start of the war.

Calling the anniversary a "tragic milestone", the United Nations High Commissioner for Human Rights Michelle Bachelet called for an independent, international mechanism to locate the missing, or identify the remains of victims.

"I also urge all parties to the conflict, and those states with influence over them, to halt arbitrary detentions and enforced disappearances," she said in a statement.

She added that people being arbitrarily detained should be immediately released.

Bachelet's office has no access to Syria but said the number of missing men, women and children was "estimated to be in the tens of thousands".

The problem of unaccounted for missing people pre-dates the outbreak of fighting in 2011.

While victims are overwhelmingly men, women are burdened with supporting themselves and their families as they try to find out what happened to a missing relative.

They face potential risks in seeking information from officials, while individuals prey on families by offering to help find their loved ones for money.

Not being able to find their relatives causes "continuing trauma for them and severely (curtails) the enjoyment of their human rights", Bachelet said.

She called on President Bashar al-Assad's regime to disclose all places of detention, provide complete lists of names and ensure formal registration of those held in these facilities.

"If an individual has died, then their body or remains should be returned to their family," she said.

Syria's war has killed more than 387,000 people and displaced millions since starting in 2011 with the brutal repression of anti-government protests.