Israeli Prime Minister Benjamin Netanyahu headed for the United States on Sunday determined to use White House talks and a key UN speech to counter "sweet talk" by arch-foe Iran.

Netanyahu has been dismissive in his response to a drive by Iran's new President Hassan Rouhani to mend fences with the international community that culminated in a historic 15-minute telephone conversation with US President Barack Obama on Friday.

The premier says Rouhani is a "wolf in sheep's clothing" whose talk of allaying Western concerns about Iran's nuclear programme is a "confidence trick" and has called on the Jewish state's US ally not to be fooled.

"I intend to tell the truth in the face of the sweet talk and charm offensive of Iran," public radio quoted him as saying before boarding a plane for Washington.

"Telling the truth at this time is essential for world peace and security and, of course, for Israel's security," he said.

Israeli media said Netanyahu had instructed government ministers to refrain from publicly commenting on the telephone call between the US and Iranian presidents for fear of complicating his White House talks on Monday.

But that has not stopped his confidants speaking out, and President Shimon Peres warned that the tone of much of the commentary was dangerously scornful of Israel's key ally.

"You can agree or disagree (with the Americans) but I don't like this scornful tone," Peres told army radio.

"Other people have brains to think too, not just us. We should talk to them and try to influence them."

After meeting Obama, Netanyahu is due to address the UN General Assembly on Tuesday, the same forum where last year he used a cartoon bomb as a prop to underline how close he believed Iran was to being able to build one.

Israel, the Middle East's sole if undeclared nuclear-armed power, remains adamant that Iran is bent on developing a weapons capability under cover of its civilian nuclear programme, something it regards as a threat to its existence.

Israeli leaders have repeatedly vowed to take military action rather than see the Islamic republic develop a bomb and have called on Washington to take tougher action against Tehran, saying they see no real change of policy under Rouhani.

Israel 'arrested Iran spy'

Underlining Israel's perception of the continuing threat posed by Iran, its Shin Bet security service announced the arrest on September 11 -- anniversary of the 9/11 attacks in the United States -- of an Iranian "spy" carrying photographs of the US embassy in Tel Aviv.

In an announcement just hours after Netanyahu left for the United States, the Shin Bet said the suspect, holding a Belgian passport, had been sent to Israel by Iran's elite Republican Guards and arrested at Tel Aviv's Ben Gurion international airport.

Former national security council chief Uzi Arad, who is close to Netanyahu, told public radio that he regretted the "softening of US policy towards Iran."

"The cracks that have appeared in President Obama's position worry me. We need to get him to be consistent with what he has said about Iran in the past."

Freesheet Israel Hayom, which backs Netanyahu, warned "the spirit of Munich is sweeping the West."

It was alluding to the 1938 Munich agreement under which Britain and France agreed to the annexation of large swathes of then Czechoslovakia by Nazi Germany in a failed bid to avert war.

But opposition Labour party leader Shelly Yachimovich warned of the dangers of a "paranoid" response to the Iran-US contacts, saying it was "vital to prevent any conflict of interest between Israel and the United States".

Alon Pinkas, former Israeli consul general in New York, said Netanyahu was making a mistake by "assuming the role of prophet of doom."

"Last year, the problem for the world was the Iranians," he said. "This year, it's us."