"And I am not 100 percent sure that we can say today - we means Mr. President and myself - that we have a common position, common opinions about Russian Federation", said Tusk, who said unity needed to be found around values like freedom and human rights and dignity.
"Today, we will take steps to keep up the momentum", adding that North Atlantic Treaty Organisation members will agree to come up with annual national plans to help them meet the target.
Secretary-General Jens Stoltenberg said Thursday that joining the US -led anti-Islamic State coalition "will send a strong political message of NATO's commitment to the fight against terrorism and also improve our coordination within the coalition". France and Germany dropped their resistance to an upgrade of NATO's role in the worldwide coalition against Islamic State on the eve of the summit.
"My feeling is that we agreed on many areas, first and foremost on counter-terrorism".
He added: "I think you can expect the president to be very tough on them and say..."
Following the meeting, he headed to the U.S. Ambassador to Belgium's residence for a working lunch with French President Emmanuel Macron before heading to the U.S. embassy in Brussels for a meet-and-greet.
"This will send a strong political message of Nato's commitment to the fight against terrorism", Nato Secretary General Jens Stoltenberg told reporters ahead of Trump's meeting with the alliance's other 27 leaders.
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Tusk and Juncker will tell the U.S. president that since last year's shock Brexit vote, the European Union is "in a completely different place" after populist anti-EU candidates lost in France and the Netherlands, a senior European Union official said.
The Republican president, midway through his first foreign trip since taking office, enjoyed favorable receptions in Riyadh and Jerusalem, where leaders lauded his harsh words for Iran.
US President Donald Trump (L) and King Philippe of Belgium speaks at a reception at the Royal Palace in Brussels, on May 24, 2017.
The visit comes as part of Trump's first global trip. The nine-day tour has included rapturous receptions in Saudi Arabia and Israel, and a polite meeting with Pope Francis in Rome.
U.S. President Donald Trump woke to a gorgeous spring morning in Brussels - and a huge flag from Greenpeace criticizing his policies. Those issues will take central stage at the G7 summit in Sicily starting Friday.
Since the election, he's had a change of heart.
Trump has mused about pulling out of the pact because he believed other countries were not paying their fair share. Secretary of State Rex Tillerson said Wednesday that "of course" the United States supports Article 5, though Trump still wants other nations to meet their obligation to spend 2 percent of their gross domestic product on defense.
The Nato summit will however be full of pomp and symbolism, with the keen-to-impress alliance showing off its new $1.2-billion headquarters and staging a flypast.