(Reuters) - Nobel Prize-winning German writer Guenter Grass has attacked Israel as a threat to world peace and said it must not be allowed to launch military strikes against Iran, in a poem that one German newspaper branded "anti-Semitic".

Grass, 84, a seasoned campaigner for left-wing causes and a critic of Western military interventions such as Iraq, also condemned German arms sales to Israel in his poem "What must be said", published in several newspapers on Wednesday.

His words were criticized in Germany, where any strong condemnation of Israel is taboo because of the Nazi-perpetrated Holocaust. Grass´s own moral authority has never fully recovered from his 2006 admission that he once served in Hitler´s Waffen SS.

"Why do I say only now ... that the nuclear power Israel endangers an already fragile world peace? Because that must be said which may already be too late to say tomorrow," Grass wrote in the poem, published in German in Sueddeutsche Zeitung.

"Also because we - as Germans burdened enough - may become a subcontractor to a crime that is foreseeable," he wrote, adding that Germany´s Nazi past and the Holocaust were no excuse for remaining silent now about Israel´s nuclear capability.

"I will not remain silent because I am weary of the West´s hypocrisy," wrote Grass, who won the Nobel Prize for Literature in 1999 for novels such as "The Tin Drum" chronicling the horrors of 20th century German history.

Israel is widely assumed to have the Middle East´s only nuclear weapons, which it neither confirms nor denies. These could be carried by Dolphin submarines it has bought from Germany.

The Jewish state has threatened to take military action, with or without U.S. support, to halt what it sees as a nuclear threat from Iran. Tehran says it is developing nuclear technology for purely peaceful purposes.

Germany said recently it would sell Israel a sixth Dolphin submarine and shoulder part of the cost, although it also cautioned its ally that any military escalation with Iran could bring incalculable risks.

"ETERNAL ANTI-SEMITE"

The poem, also published in the New York Times and in Italy´s La Repubblica, called for an international ´agency´ to take permanent control of both Israel´s nuclear weapons and Iran´s atomic plant.

The Welt newspaper called Grass "the eternal anti-Semite" in a front page article commenting on the poem, which was widely circulated in advance of its publication.

"Grass is the prototype of the educated anti-Semite who means well with the Jews. He is hounded by guilt and feelings of shame and at the same time is driven by the wish to weigh up history," the newspaper wrote on Wednesday.

Grass is for many the voice of a German generation that came of age during Adolf Hitler´s war and bore the burden of their parents´ guilt.

But Grass, who for decades urged Germans come to terms with their Nazi past, lost much of his moral authority after his belated admission in 2006 that he had once served in Hitler´s Waffen SS.

One of the most powerful organizations in Nazi Germany, the SS was first an elite force of volunteers that played a key role in the Holocaust, operating the death camps in which millions died. But by the war´s end, most were drafted and many under 18.

Grass said he was called up to join the SS as a teenager and insisted that he never fired a shot. But some critics inside and outside Germany said this explanation had come too late.

Grass made the confession shortly before publishing his autobiography "Peeling Onions" which details his war service.

(Additional reporting by Alexandra Hudson; Writing by Gareth Jones; Editing by Andrew Heavens) (© Thomson Reuters 2012. 04/04/12)