MARK COLVIN: The Israeli Prime Minister, Benjamin Netanyahu, has used his speech to the UN general assembly to attack Iran's new president and warn against working with the Iranian government.

He says president Hassan Rouhani is a "wolf in sheep's clothing".

Mr Netanyahu has threatened that Israel will act alone if it has to, to stop Iran from developing nuclear weapons. He says the world shouldn't be fooled by Mr Rouhani's charm offensive because he can't be trusted.

Mandie Sami reports.

MANDIE SAMI: There has recently been a breakthrough in relations between the US and Iran, with Mr Rouhani and the US president Barack Obama speaking on the phone together.

It was the first top-level conversation between the two countries for more than 30 years.

But speaking at the UN general assembly in New York, the Israeli Prime Minister, Benjamin Netanyahu, issued a blunt warning about the dangers of trusting Iran.

BENJAMIN NETANYAHU

Rouhani doesn't sound like Ahmadinejad. But when it comes to Iran's nuclear weapons program, the only difference between them is this: Ahmadinejad was a wolf in wolf's clothing; Rouhani is a wolf in sheep's clothing. A wolf who thinks he can pull the wool over the eyes of the international community.

MANDIE SAMI: Last year at the UN, Mr Netanyahu used a cartoon drawing of a bomb to illustrate his warning that Iran was close to developing a nuclear bomb.

BENJAMIN NETANYAHU (archive): This is a bomb. A red line should be drawn right here.

MANDIE SAMI: There were no diagrams this time but Mr Netanyahu did launch a scathing verbal attack against the Iranian leader, warning him that Israel was ready to launch military action against it.

BENJAMIN NETANYAHU: I want there to be no confusion on this point. Israel will not allow Iran to get nuclear weapons. If Israel is forced to stand alone, Israel will stand alone.

MANDIE SAMI: Tehran insists its nuclear activities are for peaceful purposes. It's due to take part in negotiations on its nuclear programme in Geneva in two weeks' time with a group of nations known as the P5+1. They include the US, Russia, China, the UK, France and Germany.

President Rouhani says Iran will bring a plan to that meeting, but he hasn't provided details.

International sanctions have hit Iran's economy hard and its leaders have made it clear they're looking for relief.

But Mr Netanyahu says, whatever promises are made, the pressure on Iran needs to be increased, not watered down:

GRAB BENJAMIN NETANYAHU: I mean, this is a ruse. It's a ploy. Why does Rouhani think he can get away with it? Because his strategy of talking a lot and doing little has worked for him in the past.

MANDIE SAMI: Immediately after Mr Netanyahu finished speaking, the Iranian envoy Khodadad Seifi used Iran's right of reply at the UN. He condemned the Israeli prime minister's remarks as "extremely inflammatory."

He also warned Iran is ready to defend itself against any Israeli attack.

KHODADAD SEIFI: The Israeli prime minister had better not even think about attacking Iran, let alone planning for that.

MANDIE SAMI: Dr Maria Rost Rublee is an international security and nuclear expert at the Australian National University. She says the US is going to have to carefully manage the relationship between Iran and Israel in the lead-up to nuclear negotiations in Geneva in two weeks' time.

MARIA ROST RUBLEE: This is really about getting the US' attention and I think that Netanyahu felt that things had gone so well for Rouhani when he was in New York for the UN meeting, and even capping the meeting off with a 15-minute telephone call with Obama, that Netanyahu felt like, you know, he needed to get the Israeli point of view out there.

Actually, I think the Israeli performance here makes it a bit easier for Obama because he can play "good cop" and he can go to the Iranians and say, "Look, you know, we need to cut a real deal here. You're going to have to make some real compromises on your nuclear program or else we're going to have to move toward the Israeli position." And I think that's probably the tactic that the US will take.

MARK COLVIN: Dr Maria Rost Rublee from the Australian National University ending Mandie Sami's report.