Paris prosecutors have opened a preliminary investigation into French government accusations that Syrian President Bashar Assad’s government has committed crimes against humanity.

France is trying to maintain international pressure on Assad, despite growing concerns in some quarters that after four years of civil war, the greater threat to Syria now comes from Islamic State extremists who have conquered swaths of the country.

The French investigation is based on photos taken by a former Syrian government photographer who fled in 2013 and focuses on atrocities allegedly committed between 2011 and 2013, the Paris prosecutor’s office said Wednesday. The graphic photos of bodies of detainees from government prisons, some of them showing gouged-out eyes or signs of torture or starvation, were shown to the U.N. Security Council last year.

The French investigation opened this month and is in its earliest stages, prosecutor’s office spokeswoman Agnes Thibault-Lecuivre said. It is unclear whether French investigators would travel to war-torn Syria, and how they might eventually bring someone to trial.

But it’s symbolically important for the French government at a time of resurgent diplomatic efforts to end the Syrian conflict.

Russia, a key Assad ally, has tried this week at the U.N. General Assembly to rally international action against the Islamic State group. On Wednesday, Russian military jets carried out airstrikes against IS targets in Syria for the first time — a move that came after President Vladimir Putin received parliamentary approval to send Russian troops to Syria.

President Barack Obama said the United States is willing to work with Russia and Iran to solve the Syrian conflict, but that Assad cannot stay in office.

France launched airstrikes this week against Islamic State targets in Syria but remains staunchly opposed to Assad because he cracked down on peaceful protests in 2011 and faces widespread opposition in his country. France doesn’t want him as any part of an eventual political solution.

“It is our responsibility to act against the impunity of these assassins,” French Foreign Minister Laurent Fabius said in a statement.

The U.N. secretary-general this week for the first time called for the civil war in Syria to be referred to the International Criminal Court. However, Russia and China vetoed a similar effort last year.

Lawyer Clemence Bectante of the International Federation for Human Rights hailed the French decision as “a strong signal” for those trying to hold someone accountable for Syria’s violence, which the U.N. estimates has killed 220,000 people.

“Recently we hear more about the crimes of Islamic State,” she said, “but this type of crime against humanity continues to be perpetrated by the regime, every day.”

(AP)