Nobel prize-winning German author Gunter Grass on Sunday launched another broadside at Israel, saying it was "an unchecked nuclear power" and an "occupying force."

Speaking on German radio station NDR, the author, already declared persona non grata in Israel over a poem saying it threatened world peace, said: "Israel is an unchecked nuclear power."

"Several United Nations resolutions go unheeded. Israel is an occupying force and for years has stolen land, evicted people and viewed them as second-class citizens. There are racist moments in Israel," he said.

"That saddens me. And that should sadden every friend of Israel, as it saddens many Israelis. And one should be allowed to speak out about it," added the author.

Grass, who said he counted himself among the "friends of Israel", defended himself against the charge of anti-Semitism, adding he believed that to refrain from criticism of Tel Aviv was "for me a new form of anti-Semitism."

Earlier this year, Grass, 84, angered Israel after publishing a piece entitled "What Must Be Said", in which he voiced fears that a nuclear-armed Israel "could wipe out the Iranian people" with a "first strike."

The publication sparked a firestorm of criticism at home and abroad and Israel has since barred him from visiting the country.

Grass acknowledged he should have specified that his criticism was directed at the government of Prime Minister Benjamin Netanyahu, not the state of Israel.

"That was careless," he admitted. "I am prepared to change this."

Grass achieved global fame with his debut novel, "The Tin Drum" in 1959, and has pressed his country for decades to face up to its Nazi past.

But he saw his substantial moral authority undermined by his 2006 admission, six decades after World War II, that he had been a member of Hitler's notorious Waffen SS as a 17-year-old.