Syria will set up commissions for justice and missing persons tasked with probing crimes committed during the rule of the Assad family, compensating victims and finding thousands of people whose whereabouts remain unknown, the presidency said.

Syria's grinding 13-year civil war has left hundreds of thousands of people, most of them civilians, dead and more than 100,000 people missing, according to United Nations estimates and human rights groups.

Former leader Bashar al-Assad was ousted by Islamist rebels Hayat Tahrir al-Sham last year in a stunning 11-day offensive, a shift met with jubilation by many Syrians who nonetheless want to see accountability for abuses suffered under the former government, including in a notorious dungeon-like prison system.

The National Transitional Justice Commission's remit is "exposing the truth about the grave violations committed by the former regime and holding those responsible accountable, in coordination with relevant authorities," said a statement from the presidency seen by Reuters on Sunday.

It did not say whether the commission would be responsible for investigating and addressing violations by other parties involved in the Syrian war, such as Islamic State.

Hassan al-Dughaim, a close adviser to the new Syrian leadership, told Reuters the transitional justice commission would focus on physical and moral reparations and national reconciliation.

There will also be trials for those legally proven to have committed crimes, but the process of gathering legal evidence that can be used in court against some individuals associated with the Assad government is difficult and complex, he said.

Former Syrian Major General Ibrahim Huwaija, accused of being implicated in the 1977 assassination of Lebanese Druze leader Kamal Joumblatt, will be among those to stand trial, al-Dughaim said.

Syrian security forces arrested Huwaija in March.

Interim President Ahmed al-Sharaa also announced the formation of a National Commission for Missing Persons, which will be tasked with investigating and uncovering the fate of missing and forcibly disappeared persons, documenting cases, creating a national database, and providing humanitarian and legal support to their families, a presidency statement said.

The UN Independent Institution on Missing Persons in the Syrian Arab Republic (IIMP) welcomed the formation of the commission in a post on social media.