French President Francois Hollande urged Europe to form a united front and provide a 'firm' response to US President Donald Trump, at a gathering of southern European Union leaders.

Lisbon: French President Francois Hollande urged Europe to form a united front and provide a "firm" response to US President Donald Trump, at a gathering of southern European Union leaders.

"We must conduct firm dialogue with the new American administration which has shown it has its own approach to the problems we all face," he said at the end of the gathering as he was flanked by the other leaders who took part.

Trump has rattled America's traditional European allies with a range of radical policy plans.

He has called Nato "obsolete", announced he would rip up a planned transatlantic trade plan and supported Britain's move to leave the EU, praising the decision as "a wonderful thing" during a meeting Friday with British Prime Minister Theresa May.

On Friday, he also signed a sweeping executive order to suspend the arrival of refugees and impose tough controls on travellers from seven Muslim countries.

During his first phone conversation with Trump late on Saturday, Hollande stressed the "economic and political consequences of a protectionist approach", adding that the principle of "acceptance of refugees" should be respected.

"Faced with an unstable and uncertain world, withdrawal into oneself is a dead-end response," Hollande was quoted as saying in an Elysee Palace statement.

Hollande had earlier told the gathering that "when he adopts protectionist measures, which could destabilise economies not just in Europe but the economies of the main countries of the world, we have to respond".

"And when he refuses the arrival of refugees, while Europe has done its duty, we have to respond."

While officially the new administration in Washington was not on the agenda, the six other European leaders who took part in the summit also alluded to Trump.

Italian Prime Minister Paolo Gentiloni said Europe was "ready, interested and willing to cooperate" with the Trump administration.

"But we are Europe, and we cherish our values," he added.

Spanish prime minister Mariano Rajoy defended the EU project, saying it had helped transform Europe into the world region with the "highest level of progress, civil rights and well being".