"I want leaders to reconfirm that the European Union sticks to the deal as long as Iran does", Tusk said.
"The role of the EU is absolutely essential", he said, "And so my appeal here in Brussels is for the the European Union to be more and more united, more and more effective, more and more present and for its voice to be more and more heard in global relations". He has made us realise that if you need a helping hand, you will find one at the end of your arm.
European leaders have scrambled to salvage the 2015 agreement in which Iran agreed to halt its nuclear program, a landmark deal that many Europeans see as essential for their security.
With member states yet to agree on how to handle the challenges, Mr Tusk called on leaders gathering in the Bulgarian capital, Sofia, to unite behind the tough line being taken by key European actors in response to the White House's actions.
"The broader US-EU security relationship is at risk", a former US envoy to the EU, Anthony Gardner, told the European Parliament on Wednesday. Despite the U.S. hesitation.
European Commissioner Dimitris Avramopoulos said the EU was willing to use legislation that would make it illegal for European companies to comply with U.S. sanctions imposed on Iran.
Tusk later addressed the issue of trade.
"Here again, unity is our greatest strength and my objective is simple - we stick to our guns".
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Tusk also demanded that Trump grant a permanent exemption to the European Union from threatened tariffs on steel and aluminum, and he rejected as "absurd" any assertion by the that such tariffs could be justified on national security grounds. Therefore US tariffs can not be justified on the basis of national security.
This move sparked discontent among the key United States trading partners, especially the EU.
"We notably discussed concrete practical solutions to make sure the European Union can continue to live up to its commitments under the deal and protect our economic operations", said Avramopoulos, the EU's migration commissioner.
Tusk said EU leaders would use the summit to discuss the Iran deal along with innovation, the European future for the western Balkans, and Trump's trade policies.
White House National Security Adviser John Bolton indicated on Sunday that the US would be willing to impose sanctions on any European companies that continue to work or do business with Iran.
The German chancellor has also voiced her anger at President Trump, and said earlier this week that while the deal was not optimal it did serve a purpse.