Israeli prime minister Benjamin Netanyahu landed in Washington yesterday vowing to “tell the truth” about Iran.

Alarmed by thawing relations between Iran and the west, Mr Netanyahu said: “I will tell the truth in the face of the sweet talk and the onslaught of smiles.”

He added: “Telling the truth is vital for the security and peace of the world and, of course, it is vital for the security of the State of Israel.”

Mr Netanyahu’s comments followed a week of conciliatory gestures between Iranian president Hassan Rouhani and the US.

Mr Rouhani addressed the UN General Assembly in New York last week, where he insisted that Iran posed no threat to the west.

This led to a 15-minute phone call between Mr Rouhani and US president Barack Obama on Friday - the first time leaders of the two nations have spoken for 34 years.

Afterwards, President Obama spoke of a “very significant shift” in Iran’s attitude and behaviour, suggesting strengthening ties between the two nations.

But Mr Netanyahu intends to warn the west against Mr Rouhani’s “smiley campaign”. He will stress the need to maintain economic sanctions on Iran and prevent Mr Rouhani from developing nuclear weapons .

He insists Iran’s tactics are a ploy to buy time and strengthen their nuclear programme .

Mr Netanyahu will meet President Obama at the White House today, before addressing the United Nations General Assembly tomorrow in New York.