The sister of the gunman behind the 2012 Toulouse massacre has left France and is suspected of joining the fight in Syria, France's interior minister said on Friday.

French authorities realized Souad Merah, 35, has left the country after a school attended by one of her children alerted them the child has not been to school in several days, AFP reported.

French sources investigating the case say Merah and her four children, aged between nine months and 14 years, took off early May for Barcelona, Spain.

From there, Merah took a flight to Istanbul, Turkey. According to French Interior Minister Cazeneuve, she didn't stop to sightsee, but immediately took another plane to Gaziantep, a Turkish city near the border with Syria.

There is a strong suspicion that she is in Syria, Cazeneuve said, AFP reported.

Souad Merah was arrested in April on suspicion that she blessed her brother, Mohammed Merah, ahead of his killing spree, in which he gunned down three French soldiers and four Jews near and in Toulouse in March 2012.

In December, police interrogated Souad Merah in Paris after she was filmed saying she was proud of her younger brother Mohammed for how he fought till the end. Filmed without her knowledge, she added: The Jews are all about massacring the Muslims. I detest them. I think well of bin Laden. I told that to the cops. I am proud of my brother, proud, proud, proud. She later issued a statement saying she condemned his actions.

The bloody three-year-old conflict in Syria has drawn thousands of Muslims to join the ranks of battalions trying to topple the regime or other fighting groups looking to conquer the region in the name of Islam.

French authorities say that more than 600 French have gone to Syria, are plotting to go or have returned, and more than 20 French have been killed in fighting. As of mid-January, a dozen French adolescents were in Syria or in transit, according to authorities.