France has launched a probe into Syrian President Bashar al-Assad's regime for alleged crimes against humanity, a judicial source said Wednesday, after world powers sparred at the United Nations over the embattled leader's fate.

Paris prosecutors opened a preliminary inquiry on September 15 into alleged crimes committed by the Syrian government between 2011 and 2013, the source told AFP.

The French investigation is largely based on evidence from a former Syrian army photographer known by the codename "Caesar," who defected and fled the country in 2013, bringing with him some 55,000 graphic photographs.

Foreign Minister Laurent Fabius said France had a "responsibility" to take action.

"Faced with these crimes that offend the human conscience, this bureaucracy of horror, faced with this denial of the values of humanity, it is our responsibility to act against the impunity of the assassins," Fabius said in a statement sent to AFP.

While Assad is unlikely to ever take the stand in a French court, the inquiry could add to political pressure on the Syrian leader in the midst of a diplomatic row between the West and Russia and Iran over his fate.

The Syrian conflict has taken centre stage at the UN General Assembly, where US President Barack Obama and his Russian counterpart Vladimir Putin have clashed over how to bring an end to Syria's civil war.

'Assad's secret killings'

On Tuesday, Obama said removing Assad was a vital step to defeating Islamic State jihadists, who have taken advantage of the chaos in Syria to bring large parts of the country and neighbouring Iraq under its rule.

Syria's four-year war has killed more than 240,000 people and Western diplomats have accused Damascus of killing more Syrians than IS by dropping barrel bombs – charges the government denies.

The brutal conflict has also displaced millions of people, a key driver behind Europe's refugee crisis.

The photographs that Caesar brought out of Syria show people with their eyes gouged out, emaciated bodies, people with wounds on the back or stomach, and also a picture of hundreds of corpses lying in a shed surrounded by plastic bags used for burials.

Entitled "Assad's secret killings," the dossier is being used by international bodies including the UN as part of an investigation into the regime's role in "mass torture".

The Syrian government has branded the report "political".

These "thousands of unbearable photos, authenticated by many experts, which show corpses tortured and starved to death in the prisons of the regime, demonstrate the systematic cruelty of the Assad regime," Fabius told AFP.

The inquiry will be led by France's war crimes body but Fabius also called on the UN and particularly its International Commission of Inquiry on Syria to press on with their investigations.

(AFP)