The brother of Islamic extremist Mohamed Merah has been convicted of complicity to murder for his involvement in a horrific 2012 attack in southern France that killed seven people.

An appeals court in Paris on Thursday raised Abdelkader Merah's sentence from 20 years to 30 after finding him guilty of the additional charge. He had previously been convicted of ties to terrorism but acquitted of conspiracy to murder, leading prosecutors to lodge an appeal.

Mohamed Merah killed three soldiers before opening fire on a Jewish school, slaying a rabbi, his two young sons and a schoolgirl, in the Toulouse area in March 2012.

He died days after the killings following a standoff with France's police special forces.

It was France's deadliest school shooting and the bloodiest attack on Jewish targets in decades.