A Paris appeals court has upheld the validity of an arrest warrant issued for the Syrian leader Bashar al-Assad over alleged complicity in war crimes committed during the country’s civil war, according to lawyers.

The judges ruled on Wednesday that the warrant, which French anti-terrorism prosecutors had sought to annul on the grounds that al-Assad enjoys immunity as a serving head of state, remains in force.

“This is a historic decision. It’s the first time a national court has recognised that a sitting head of state does not have total personal immunity” for their actions, said plaintiffs’ lawyers Clemence Bectarte, Jeanne Sulzer and Clemence Witt.

Mazen Darwish, director of the Syrian Center for Media and Freedom of Expression, said the decision “shows that there is no immunity when we are talking about crimes against humanity and using chemical weapons against civilians”.

The case against al-Assad is a particularly high-profile example of victims of the country’s devastating civil war seeking accountability for government forces that took part in atrocities.

An arrest warrant was issued for al-Assad, his brother Maher, and two Syrian generals in November for alleged complicity in war crimes, including chemical weapons attacks on the cities of Douma and East Ghouta in 2013.

With al-Assad maintaining a large degree of control over Syria, devastated by years of fighting during which his forces were accused of atrocities against civilians, Syrians who fled the country and settled in Europe have launched legal efforts to hold members of the Syrian military and government to account.

The August 2013 chemical attacks killed more than 1,000 people and injured thousands more, but international condemnation produced little change in the Assad government’s prosecution of the war.

In May, anti-terror prosecutors contested the arrest warrant against al-Assad, maintaining that acting heads of state enjoy absolute immunity. The prosecutors did not challenge the arrest warrant for al-Assad’s brother or the Syrian generals Ghassan Abbas and Bassam al-Hassan.

Those named in the warrants can be arrested and brought to France for the investigation, an unlikely outcome that advocates nonetheless say sends a message of accountability at a time when the Assad government has begun to come back in from the cold after years of being shunned by regional governments and organisations.