* 15-year old's father says his son has been brainwashed
* 700 French and residents believed fighting against Assad
* Fears grow nationals will return to plot attacks at home
By Julie Rimbert and John Irish
TOULOUSE/PARIS, Jan 17 (Reuters) - Two French 15-year-olds have left France to join Islamist militants in the fight against Syrian President Bashar al-Assad, one of their fathers and the Toulouse prosecutor said on Friday.
"I've informed the Paris anti-terrorist prosecutor because the two boys made their intention clear that they would travel to Syria via Turkey," prosecutor Michel Valet told Reuters.
French officials say they are increasingly worried about their own nationals travelling abroad to fight in Syria's civil war and one day returning to plot attacks at home.
The father of Hakim, one of the two boys from the southwestern city of Toulouse, told BFM TV his son had left a note on Jan. 6. explaining he was going to join the Jihad, or Holy War, against Assad.
He said Hakim left cash behind to cover the cost of a plane ticket to Turkey which he bought with his father's credit card.
According to the father, who did not give his name, the boy called the family three days ago to say he was in danger in Syria and that he would not call again for a month.
He added that if he did not call by then the family should assume he was dead and they would next meet in paradise.
"He has been brainwashed on the Internet," the father said.
As the Syrian civil war enters its third year, more and more Europeans are joining the rebellion, according to the European Union, which in May recommended better tracking of social media to spot foreign fighters.
President Francois Hollande said on Wednesday about 700 French nationals and residents had travelled to fight in Syria.
He has made clamping down on violent cells and self-radicalised "lone-wolf" operators planning domestic attacks a priority since a Toulouse-based al Qaeda-inspired gunman Mohamed Merah shot dead seven people in March 2012.
France has been on heightened security alert since January 2013, when it intervened in Mali to repel al Qaeda-linked rebels who had seized control of the north of the former French colony.
Three people were arrested in June as part of a group suspected of sending Islamist fighters to Syria.
Two brothers from Toulouse were killed recently in Syria, one in a suicide bombing, after appearing on a video urging Hollande to convert and Muslims to join the war.
"Since the Merah case, we've had several of cases of young people travelling to Syria although it is not exclusive to Toulouse," Prosecutor Valet said.
Speaking to reporters on Friday Interior Minister Manuel Valls said 20 French jihadis had died in Syria.
"It shows the magnitude of the phenomenon in France and Europe," he said. (Reporting by John Irish; editing by Andrew Roche)