French anti-terrorist authorities have been alerted after the Chief Rabbi of Lyon was sent a threatening letter accompanied by images of Jewish children being marched to World War II concentration camps.

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The Rabbi, Richard Wertenschlag, told AFP on Wednesday he had brought the missive to the attention of the authorities because of its particularly offensive and menacing content.

"One could brush it off as the work of one rogue individual but we have to envisage the worst scenario -- prevention is better than a cure," he said.

In the letter, which was sent to Lyon's main synagogue on August 10, the author complained: "More and more frequently we are having ideas imposed on us
that have as their goal to apologise for the Jew, the so-called Shoah (holocaust), the evil Palestinians.

"From now on we will punish a Jew ... each time that you go on television to complain."

Referring to the enclosed photographs, it added that they "prove that the Jews were well treated in Germany, fed and lodged with summer camp for the
children."

The Rabbi said the letter was the first explicitly threatening one he had ever received but added that anti-Semitic or anti-Zionist correspondence was
not unusual.

"It relates to a certain climate," he said. "You see many expressions of a new anti-Semitism that is surfacing."