The UN General Assembly on Thursday created an independent body to "clarify" the fate of thousands of people who remain missing in Syria since war broke out in 2011, overriding objections from Damascus.

According to non-governmental groups, some 100,000 people have disappeared since the start of a brutal crackdown on anti-government protests. More than half a million people have been killed.

"After 12 years of conflict and violence in the Syrian Arab Republic, little progress has been achieved in alleviating the suffering of families by providing answers as to the fate and whereabouts of all missing persons," said the resolution, which passed with 83 votes in favor, 11 opposed and 62 abstentions.

The Independent Institution on Missing Persons in the Syrian Arab Republic will "clarify the fate and whereabouts of all missing persons" in the country and "provide adequate support to victims, survivors and the families of those missing."

But Syria, along with Russia, China and Iran, objected to the new body, saying it had not been consulted on the resolution.

"This draft clearly reflects flagrant interference in our internal affairs and provides new evidence of the hostile approach being pursued by certain Western States against Syria," ambassador Bassam Sabbagh said, pointing in particular to the United States.

He rejected the "bizarre mysterious mechanism with no precise definition of the concept of missing persons, a mechanism that is not time limited or geographically bound" and warned it could be used to target UN member states, particularly developing countries.

The resolution calls for the UN secretary-general, the UN high commissioner for human rights and other relevant parties to define the new organization's mandate in the next 80 days.

"A much-needed initiative! Families have right to know fate & whereabouts of loved ones, to help heal society as a whole," the UN Human Rights Office said on Twitter.

Human Rights Watch said the new organization needs to have the right tools at its disposal to do its job.

"UN member countries should ensure that this new institution has the staff and resources necessary to determine what happened to so many thousands of people who vanished during Syria's 12 years of conflict," Louis Charbonneau, the group's UN director, said in a statement.

"The people of Syria deserve no less."

Thursday's vote came after UN chief Antonio Guterres called for the formation of such a body in a report in August, saying families should not have to go through the trauma of investigating the whereabout of missing relatives themselves.

"As a family association representative said, 'Just imagine looking through leaked videos of massacres (on social media) to see if your loved one is among the decapitated, mutilated bodies over and over, desperately conducting your own search.'"

"Women-led households in particular should not have to put themselves at risk to search for their missing relatives, nor impoverish themselves in an economic environment that is already dire and where survival is precarious, in order to claim basic rights."