French President Nicolas Sarkozy on Monday declared a shooting that left four dead outside a Jewish school to be a "national tragedy" and vowed to find the killer.

Visiting the scene of the killings in the southern city of Toulouse, Sarkozy announced a minute of silence in all French schools on Tuesday and said the state would throw its entire weight behind the investigation.

"We should not back down in the face of terror," he said, his voice cracking as he paid tribute to the parents and school authorities. "Barbarism, savagery, hate must not win. The Republic is much stronger than that.

"You cannot murder children like this on the territory of the Republic without being held to account," he said. "Today is a day of national tragedy."

"I want to say to all the leaders of the Jewish community, how close we feel to them. All of France is by their side," he said, adding that all Jewish and Muslim schools in the region would see increased security measures.

Sarkozy said his interior minister, Claude Gueant, would remain in Toulouse for as long as it takes to get to the bottom of a series of three shootings in the Toulouse area that he now believes are linked.

Children aged three, six and 10, and a 30-year-old religious education teacher were mown down as they arrived for class at the Ozar Hatorah school in Toulouse, by a killer who arrived on a motorbike with two pistols.

On March 11 an off-duty French paratrooper was shot dead in a similar attack, by an assailant also mounted on a motorbike and wielding a large .45 calibre automatic pistol.

On Thursday three more paratroopers, based this time in nearby Montauban, were shot while standing at a cash machine near their barracks. Two were killed and one badly wounded, by an attacker using the same gun.

"We are struck by the similarities in the method used, but we must await the result of the judicial inquiry," said Sarkozy. "He who did this should know that everything will be done to bring him to account."

Anti-terror police have taken over the inquiry and the French presidential election race has been effectively suspended, with both right-winger Sarkozy and his Socialist rival Francois Hollande deciding to visit the school.