Tusk told a news conference in Sofia, Bulgaria, on Wednesday that “Europe should be grateful” to Trump since he has "rid Europe of all illusions" with the trade dispute and withdrawal from the Iran nuclear agreement, officially known as the Joint Comprehensive Plan of Action (JCPOA).
Tusk said the EU had to be united to face "a new phenomenon - the capricious assertiveness of the American administration."
"Looking at the latest decisions of President Trump, someone could even think that with friends like this, who needs enemies?" Tusk said, Presstv Reported.
He said the EU needed a united front on the Iran nuclear deal, noting, “We must maintain it."
Tusk said that the EU needed to protect European companies against US decisions, but also sought ways to address Iran's ballistic program and regional role.
Trump announced on May 8 that Washington was walking away from the JCPOA, which was reached between Iran and the five permanent members of the UN Security Council -- the US, Britain, France, Russia and China -- plus Germany in 2015.
Trump also said he would reinstate US sanctions on Iran lifted by the accord and impose "the highest level" of economic bans on the Islamic Republic.
The deal lifted sanctions in 2016 in return for Tehran limiting its nuclear program.
The remarks come as in their first meeting since the US withdrawal from the JCPOA, EU leaders were expected to meet to explore options on Wednesday for keeping the Iran nuclear deal alive and protecting their reviving economic cooperation with Tehran.
The head of the European Commission, the bloc’s executive, Jean-Claude Juncker, will also present options to shield European investments in Iran and the reviving economic cooperation.
The options include protecting European companies dealing with Iran from US sanctions, allowing the European Investment Bank to invest in Iran and coordinating euro-denominated credit lines from EU states.
“It’s the leaders’ first face-to-face meeting since Trump’s announcement. It’s too early for specific decisions, they have to see where they are,” said a senior EU diplomat. “We want to do this, that’s for sure. But it’s still in the making, we don’t have all the answers yet. It will take a bit of time.”
Iran has reiterated it would remain in the JCPOA for now, pending negotiations with the other signatories in the coming weeks before making a final decision on its future role in the agreement.
Iranian Foreign Minister Mohammad Javad Zarif met EU foreign policy chief Federica Mogherini and his counterparts from Britain, France and Germany in Brussels on Tuesday and tasked their experts to come up with measures for a meeting of their deputies in Vienna next week.
Russia's Deputy Foreign Minister Sergei Ryabkov said on Wednesday that Moscow supported a European Union proposal to hold a meeting on the Iran nuclear deal in Vienna next week, Interfax news agency said.
Before visiting Europe, Zarif first traveled to China and later Russia, the two other signatory nations that have supported the JCPOA, on the first leg of his diplomatic mission.
Russian President Vladimir Putin will also host Merkel on Friday to discuss, among other issues, the Iran deal.
Juncker hopes EU will find joint response to US exit from Iran deal
Separately on Wednesday, Juncker said he hoped that EU leaders would agree over diner later in the day on a joint response to the US withdrawal from the Iran nuclear deal.
Speaking after a meeting with UN Secretary General Antonio Guterres, Juncker admitted, however, that the EU's means to keep the deal alive were limited.
Guterres, for his part, said he did not expect the Security Council to re-impose sanctions that were lifted when the deal was implemented.
Earlier on Tuesday, Guterres met with Tusk at the European Council.