European leaders are troubled by Trump's "America first" rhetoric and inconsistent statements on North Atlantic Treaty Organisation and the EU.
On Wednesday evening in Sofia, EU leaders spent several hours over dinner discussing how to protect the EU's interests, after two recent decisions by United States president Donald Trump: scrapping a nuclear deal with Iran and potentially sanction European companies doing business there and threatening to impose tariffs on steel and aluminium from 1 June.
Tusk later addressed the issue of trade.
"Looking at the latest decisions of President Trump, someone could even think with friends like that who needs enemies", Tusk told reporters.
But thanks to Trump, he said, "we have got rid of all illusions. He made us realize that if you need a helping hand, you will find one at the end of your arm".
'This means a permanent exemption from USA tariffs on aluminium and steel if we are to discuss possible trade liberalisation with the US.
Tusk also called for the European Union to unite "economically, politically and militarily, like never before".
"Besides the traditional political challenges such as the rise of China or the aggressive stance of Russian Federation, we are witnessing today a new phenomenon: the capricious assertiveness of the American administration", Tusk said.
Listing Europe's traditional challenges, ranging from the expanding power of China to the belligerence of Russia, Tusk said: "We are witnessing today a new phenomenon, the capricious assertiveness of the American administration. This is the only real alternative".
The EU's hopes have been complicated by the fact that European companies would also be exposed to United States sanctions imposed on anyone doing business in 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. "I want the European Commission to be given a green light, so it is ready to act when European interests are affected", he added.
Merkel however said that Europe had no choice but to stick with the ties that have bound it to Washington since World War II.
'We will decide on the best way forward in EU-US trade relations.
Mr Tusk said he wanted the 28 leaders over the next 24 hours to reconfirm that the European Union would stick to the deal as long as Iran did.
The 28 anxious EU leaders are gathering in the Bulgarian capital for discussions over dinner on Wednesday on how to salvage the nuclear deal and European business dealings with Iran from Trump's sanctions and how to avoid a trade war in an escalating tariff dispute with the United States.
"I will propose we stick to our guns", Tusk said.
"The EU will continue fighting for a rules-based worldwide system despite recent decisions on climate change, tariffs and Iran", the source said after the leaders met over dinner in Sofia.
"The way to address will give answer to the broader question on how Europe should deal with the Trump administration", a senior European Union official had noted before the meeting.