Western plans for retaliatory action against Syria for an alleged chemical weapons attack are "difficult to develop", the French government spokeswoman said Thursday.

"The international community must find a riposte that is adapted to the situation," Najat Vallaud-Belkacem said on France 2 television.

French prosecutors are investigating alleged U.S. spying under the PRISM surveillance program following complaints by two human rights groups, sources close to the case said Wednesday.

They launched an investigation on July 16 into fraudulent access to personal data and personal correspondence following complaints by the International Federation of Human Rights and the League of Human Rights, the sources said.

The French parliament will hold an emergency session to debate the Syria crisis on September 4, minister Alain Vidalies said Wednesday.

The announcement comes as France and its allies weigh a potential military intervention in Syria following an alleged chemical weapons attack last week in the Damascus suburbs that the West blames on the regime.

French President Francois Hollande will meet with the head of Syria's main opposition group on Thursday, the presidency announced Tuesday.

The talks with Ahmad al-Jarba will take place just over a week after an alleged deadly chemical attack took place in the suburbs of Damascus, prompting increasingly strident calls for action against the Syrian regime.

French President Francois Hollande on Tuesday called on the United Nations and African Union to "take charge of the situation" in the violence-wracked Central African Republic.

The former French colony has been beset by reports of widespread rape, recruitment of child soldiers, weapons proliferation, huge population displacement and severe malnutrition since a coup in March.

President Francois Hollande said Tuesday that France was "ready to punish" those behind last week's alleged deadly chemical attack in Syria, adding that the conflict threatens "world peace".

"France is ready to punish those who took the vile decision to gas innocent people," he said in a televised speech, pointing the finger of blame at President Bashar Assad's regime.

France will not "shirk its responsibilities" in Syria, a source in the French presidency said Tuesday ahead of a speech by President Francois Hollande on the Syrian crisis.

The source said the "massive use" of chemical weapons is "unacceptable", adding that there was no doubt that President Bashar Assad's regime had used them in a deadly attack last week on a Damascus suburb.

The French foreign minister said Monday there will be a "proportionate response" to the alleged chemical weapons attack in a Damascus suburb that he blamed on Syria's government.

"It will be negotiated in coming days," Foreign Minister Laurent Fabius told Europe 1 radio on Monday.

French President Francois Hollande told his U.S. counterpart Barack Obama on Sunday that "everything was consistent" with the conclusion that Damascus was behind last week's suspected chemical attack.

"The head of state condemned the use of chemical weapons in Syria and indicated that everything was consistent with designating the Damascus regime as the perpetrator of these unacceptable attacks," the French president's office said in a statement.

French President Francois Hollande said Sunday that evidence indicated Syrian President Bashar Assad's regime had carried out chemical weapons attacks on its own people last week.

Hollande said there was "a body of evidence indicating that the August 21 attack was chemical in nature, and that everything led to the belief that the Syrian regime was responsible for this unspeakable act".