Iran will ban Americans from entering the country in response to President Donald Trump's “insulting” order restricting arrivals from Iran and six other Muslim states, the foreign ministry said on Saturday.

“The Islamic Republic of Iran... has decided to respond in kind after the insulting decision of the United States concerning Iranian nationals” until the measure is lifted, the ministry said in a statement carried by state television.

Trump on Friday signed a sweeping executive order to suspend refugee arrivals and impose tough controls on travellers from Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen.

Iran's foreign ministry called the decision “illegal, illogical and contrary to international rules”.

It said its own ban on US nationals would continue until the American measure was lifted.

The ministry said said it had ordered Iranian diplomatic missions to help Iranians who had been “prevented from returning to their homes and places of work and study” in the United States.

Travel agents in Tehran said that foreign airlines had begun barring Iranians from US-bound flights.

French leader chides US populism, urges European unity

French President Francois Hollande on Saturday urged Europe to present a united front against populist movements which, he said, are being encouraged by political developments in the United States.

“Europe is facing a moment of truth,” Hollande said. “The issue is populism. What we are hearing from the US encourages populism and even extremism. They are saying that Europe should not take immigrants, shouldn't stay together, not believe in climate change.”

European Union countries should stick to their principles and defend their interests and stand firm in talks with US President Donald Trump, Hollande said.

“Europe should be true to itself. It should guide itself according to its values, its principles, and its interests,” Hollande said on the sidelines of an informal meeting with six other EU leaders in Lisbon, Portugal. “We should engage in discussions (with the US) that sometimes should be very firm.”

“And as long as there are statements from the US president about Europe, when he speaks about the model of Brexit for other countries, when the US president talks about climate change ... saying he's not convinced of it, we should respond to him. When he takes protectionist measures, we should respond to him. When he destabilizes the economies of other countries, not only European ones, we should respond to him. When he rejects the arrival of refugees, while Europe has done its duty, we should respond to him,” he told reporters.

Italian premier Paolo Gentiloni said the EU will “find a way to work with the US administration” but added that the bloc should remain true to its core values including human rights and anti-protectionism.

The EU leaders emphasised their commitment to the European Union, in the wake of Britain's vote to leave the bloc.

“We believe that in a world confronted with growing uncertainties and instability, we will be stronger by acting together,” they said in a joint statement. “Weakening Europe is not an option.” However, they said they hoped “to have the United Kingdom as a close partner of the EU.”