Secretary of State Hillary Clinton had said the talks would open April 13 in Istanbul. But Iran later said that Turkey was not an acceptable host after the NATO member cut oil imports from Tehran in response to US pressure.

"We are still expecting this to take place next week. But there's certainly some degree of urgency" in finalizing details, State Department spokesman Mark Toner told reporters.

"It was our expectation that this was going to be in Istanbul," he said. "It's not for us to say one place over another, but it's important that we start to nail this down."

Toner said EU foreign policy chief Catherine Ashton was working with Iran to determine the details of the talks. Iraq has said that Iran approached it about holding the talks in Baghdad.

Iran last held talks with the six powers -- Britain, China, France, Germany, Russia and the United States -- in January 2011 with no results.

President Barack Obama's administration has been eager to resume diplomacy to avoid a military confrontation, amid speculation that Israel would decide to strike Iran if it determines that Tehran is going ahead with a nuclear bomb.

Clinton said Wednesday the United States wanted a "peaceful resolution" but that "the time for diplomacy is not infinite and all options remain on the table to prevent Iran from acquiring a nuclear weapon."

Iran says its sensitive uranium work is for peaceful purposes. US intelligence assessments have not determined that the clerical regime is building a nuclear bomb.

"The shameful comparison that Gunter Grass made between Israel and Iran, a regime which denies the Holocaust and calls for Israel's destruction, says very little about Israel and a great deal about Mr Grass," he said in a statement.

"It is Iran, not Israel, which presents a threat to the world's peace and security. It is Iran, not Israel which threatens the destruction of other states," he said.

"It is Iran, not Israel, which supports the massacre carried out by the Syrian regime on its citizens."

Israel's foreign ministry described the Grass poem as "pathetic."

"The transition of Grass from fiction to science fiction is in very poor taste; his poem is pathetic and totally lacking grace," ministry spokesman Yigal Palmor told AFP.

Israel's Haaretz daily took the same tone, with historian Tom Segev writing that the German writer, who won the Nobel literature prize in 1999, was "more pathetic than anti-Semitic."

"The comparison between Israel and Iran is unfair because, unlike Iran, Israel has never threatened to wipe another country off the map," Segev wrote, referring to 2005 comments by Iranian President Mahmoud Ahmadinejad widely translated into English as meaning the Jewish state should be "wiped off the map."

In Grass's poem "What must be said," the 84-year-old longtime leftist activist wrote of his concern that Israel "could wipe out the Iranian people" with a "first strike" due to the threat it sees in Tehran's disputed nuclear programme.

"Why do I only say now, aged and with my last ink: the atomic power Israel is endangering the already fragile world peace?" reads the poem, which was published in the daily Sueddeutsche Zeitung.

Grass, author of the renowned anti-war novel "The Tin Drum," sparked outrage in 2006 when he revealed, six decades after World War II, that he had been a member of the notorious Waffen SS.

Israel, the sole if undeclared nuclear power in the Middle East, has said it is keeping all options open for responding to Iran's nuclear programme, which it says is aimed at securing nuclear weapons, posing an existential threat to the Jewish state.

Iran has consistently denied that its sensitive nuclear work is aimed at making weapons.