European Council president Donald Tusk has derided the actions of the United States under Donald Trump as "capricious", noting: "Someone could even think: 'With friends like that, who needs enemies?'"

As trans-Atlantic problems mount, Mr Tusk is leading an EU summit in Bulgaria to address the outstanding issues with the Trump administration which centre on Washington pulling out of the Iran nuclear deal and its threat to impose punitive trade sanctions on the 28-nation EU.

Listing the traditional problems facing Europe, ranging from the expanding power of China to the belligerence of Russia, Mr Tusk said: "We are witnessing today a new phenomenon, the capricious assertiveness of the American administration."

In unusually harsh terms, Mr Tusk said Europe could no longer count on automatic help from Washington as Western allies in an increasingly troubled world.

Hours before the 28 leaders were to sit down for their summit dinner in Sofia, Mr Tusk said: "Frankly speaking, Europe should be grateful to President Trump because thanks to him we have got rid of all illusions. He has made us realise that if you need a helping hand, you will find one at the end of your arm."

Mr Tusk has long been critical of the American president, even famously saying before the 2016 US elections that one Donald in international politics was enough.

Europe exemplifies the use of multilateral international negotiations, to settle everything from global trade rules to the threat of Iran developing nuclear weapons. Mr Trump's actions to upset decades-old diplomatic conventions with fully-capitalised tweets have increasingly stunned the EU.

Under Mr Trump, the US has walked out of the global Paris climate deal, is threatening to impose punitive tariffs based on national security on an ally like the EU, and has stirred Middle East politics by pulling out of the Iran nuclear deal and moving the US embassy in Israel to Jerusalem.

"We need a united European front," Mr Tusk told reporters about efforts to salvage the Iran deal. "I want leaders to reconfirm that the EU sticks to the deal as long as Iran does. The deal is good for European and global security."

He also warned that EU nations should prepare to confront US sanctions that could hit their firms doing business with Iran.