Previously in March Tusk offered a more conciliatory stance on the economic differences between the U.S. and European Union, urging Trump to "go back" to trade talks instead of waging a "trade war".
The EU's 28 leaders are due to discuss two challenges that Trump has confronted them with: his decision to pull out of the Iran nuclear deal, and the US threat to impose tariffs on metal imports.
Looking at the latest decisions of Trump, someone could even think: with friends like that, who needs enemies?And he exhorted European leaders not to be reliant on Washington.
Merkel however said that Europe had no choice but to stick with the ties that have bound it to Washington since World War II.
On Saturday, Merkel told Europeans that the USA had made potential peace in the Middle East "even more difficult" and warned that "the harder the problem is, and the easier the solution is claimed to be, the more suspicious and critical everyone should be". The shifting desires make it almost impossible to negotiate with the White House, many diplomats say, because they can not strike a bargain to get close to what Trump wants when he doesn't know it himself. "But at the same time, we must be prepared for those scenarios, where we will have to act on our own", said Tusk, a former Polish prime minister.
Earlier this month, US President Donald Trump announced that his country would exit the 2015 nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA), and reinstate the previously lifted sanctions on Iran.
Lyft Follows Uber in Ending Forced Arbitration For Sexual Harassment Claims
Uber chief executive Dara Khosrowshahi said the move was "an important step forward in our commitment to safety and transparency". She said this is the "beginning of a longer process needed to meaningfully improve safety".
Tusk's comments came ahead of a gathering of heads-of-state of the European Union in the Bulgarian capital Sofia on Wednesday.
"But frankly speaking, Europe should be grateful to President Trump". Thanks to him we 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.
"The deal is good for European and global security, which is why we must maintain it".
With Trump's cabinet being increasingly populated by the likes of National Security Adviser John Bolton and Secretary of State Mike Pompeo, who have long pushed for regime change in Iran, the U.S. position is becoming more at odds with those of Europe, Russia, and China.
Tusk's comments underscore the rift between the trans-Atlantic allies since Trump came to power on a platform of "America First". The contrast has stirred up even more European anger toward Trump's foreign policy, and the Gaza violence is another mark on the agenda for European leaders to discuss during their day and a half of meetings that starts Wednesday.
"The EU and United States are friends and partners, therefore U.S. tariffs can not be justified on the basis of national security", Tusk said. There appears to have been little progress in those discussions. "It's a moment of truth for Europe", he said. He said: "We are witnessing today a new phenomenon: the capricious assertiveness of the American administration".