PARIS — 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 of mutilated corpses taken by a former Syrian officer who fled in 2013 and focuses on atrocities allegedly committed between 2011 and 2013, the Paris prosecutor’s office said Wednesday. The Foreign Ministry requested the probe.

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The investigation, opened this month, 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, who might eventually be charged, and how they would be brought 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 UN General Assembly to rally international action against the Islamic State group, and some Western voices are softening their positions against Assad.

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.

“In the face of these crimes that attack the human conscience, this bureaucracy of horror, in the face of the negation of the values of humanity, it is our responsibility to act against the impunity of these assassins,” French Foreign Minister Laurent Fabius said in a statement.

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

Copyright 2015 The Associated Press.