Iranian leader Hassan Rouhani Thursday called on Israel to admit it has a nuclear bomb ahead of a landmark meeting between Iranian and western foreign ministers.

Rouhani also said he believed a deal could be struck with the international community on his own country's controversial nuclear drive within three to six months.

The Iranian president spoke at a UN nuclear disarmament conference just before Iran's Foreign Minister Javad Mohammad Zarif was to hold landmark talks with US Secretary of State John Kerry and ministers from Britain, China, France, Germany and Russia.

The meeting was called to discuss western allegations that Iran is seeking nuclear weapons capability and was to be one of the highest level Iran-US encounters since the 1979 Iranian Revolution.

Rouhani said Iran's arch-foe Israel should join the Non-Proliferation Treaty, which aims to contain the spread of nuclear weapons. Israel has never declared a nuclear arsenal, but is widely assumed to have several bombs.

Rouhani, speaking as current leader of the Non-Aligned Movement, highlighted the failure of attempts to organize a Middle East nuclear free zone.

"Israel, the only non-party to the Non-Proliferation Treaty in this region, should join thereto without any further delay," Rouhani told the meeting.

He said "all nuclear activities in the region" would then be subject to International Atomic Energy Agency safeguards.

Iran is in dispute with the UN's atomic watchdog, which says it has still not given definitive proof that its nuclear program is for peaceful purposes.

Iran denies it seeks a bomb.

Kerry met early Thursday with his Chinese counterpart Wang Yi and the two men agreed "that Iran should cooperate and should respond positively to the offer on the table," a US official told reporters.

The group, dubbed the P5+1, made a new offer to Iran earlier this year, before Rouhani's election, on how to overcome a current stalemate in the nuclear dossier.

It is believed to have offered an easing of the international sanctions which have crippled the Iranian economy, in return for a slow down in Iran's controversial uranium enrichment program.

Western officials say they are still waiting for a response.

"No nation should possess nuclear weapons, since there are no right hands for these wrong weapons," insisted Rouhani, who also met Japan's Prime Minister Shinzo Abe on Thursday.

"As long as nuclear weapons exist, the risk of their use, threat of use and proliferation persist. The only absolute guarantee is their total elimination," he said.

Rouhani told the Washington Post he wants a quick deal on the nuclear standoff adding he has the full backing of Iran's Supreme Leader Ayatollah Ali Khamenei.

"The only way forward is for a timeline to be inserted into the negotiations that's short -- and wrap it up," said Rouhani.

"If it's three months, that would be Iran's choice, if it's six months, that's still good. It's a question of months not years."

Zarif will be the first Iranian minister to sit down with western counterparts to discuss Iran's nuclear program.

German Foreign Minister Guido Westerwelle also discussed the Iranian nuclear program with Wang Thursday, and both agreed the new Iranian government must be taken at their word, and called for "swift progress."

While US officials say no bilateral talks are planned between Kerry and Zarif, an unofficial encounter is possible.

"We're going to have a good meeting I'm sure," Kerry said earlier Thursday.

Western diplomats say the meeting, being hosted by EU foreign policy chief Catherine Ashton, will give Iran a chance to show there is substance behind Rouhani's statements that he wants to end the nuclear showdown.

Zarif said on his Twitter account: "We have a historic opportunity to resolve the nuclear issue," if world powers adjust to the "new Iranian approach."

The United States sought a meeting between President Barack Obama and Rouhani on the sidelines of the UN assembly. But Iran said it was too difficult.

Zarif said in remarks published by Iran's official IRNA news agency that a meeting between the presidents "would have been a good beginning."

"A meeting is not an end in itself, nor is it ruled out... President Rouhani has no problem in principle" with meeting Obama, he said.

He called Obama's remarks at the United Nations "more moderate" than in the past.

"If that is the basis of a new political will to solve misunderstandings and ease Iranian concerns, this would be more important than a meeting," he said.

Obama and Rouhani made cautious overtures to each other in their UN speeches on Tuesday.

Rouhani said Iran poses "absolutely no threat to the world," and condemned the international sanctions against his country.

Obama said there was a basis for "a meaningful agreement." But he stressed: "To succeed, conciliatory words will have to be matched by actions that are transparent and verifiable."