The rebel alliance that overthrew the Assad government in Syria has vowed to find and punish senior officials who served in the previous regime.

“We will not relent in holding accountable the criminals, murderers, and security and military officers involved in torturing the Syrian people,” Ahmed al-Shara, the leader of the alliance, formerly known by his nom de guerre Mohammed al-Jolani, said in a statement posted on the Telegram social media app.

He gave no details about how the rebel group would pursue that course, or of any judicial process, which human rights experts say is essential to helping Syria move forward.

The rebel alliance, Hayat Tahrir al-Sham, had earlier announced an amnesty for the rank and file of the government of former president Bashar al-Assad, but al-Shara said this did not extend to senior officials.

His comments came amid fear that people who supported the fallen government could face retribution.

Hayat Tahrir al-Sham, an Islamist group that was once linked to al Qaida, has tried to gain international legitimacy by eschewing global jihadist ambitions and focusing on organised governance in Syria. But it has also come under criticism for using authoritarian tactics and cracking down on dissent.

Human rights groups say that more than half a million people including around 200,000 civilians, died in Syria’s civil war, which began in 2011, but attention has focused on atrocities committed by Assad’s government.

The whereabouts of around 136,000 people who were arrested arbitrarily by the Assad regime is unknown and in many cases the government did not acknowledge that people had been detained, leaving family members in the dark, according to the Syrian Network for Human Rights.

In addition, at least 15,000 people were tortured to death by the government, the network said.

Human rights experts say a judicial process is necessary and that the options for the rebel alliance include building cases in national, foreign and international courts. But they caution that it would take time to set up a national system capable of rendering justice and it can only be done once security in the country has been established.

Human rights groups in Syria, working with international organisations, have spent more than a decade documenting evidence of crimes committed in the country, particularly in the regime’s notorious prison network.