Referencing Trump's impending summit with Putin, Tusk on Tuesday said, "It is always worth knowing: who is your strategic friend?"
But NATO chief Jens Stoltenberg pointed out that military spending had been rising in Europe since 2014 and members were making efforts to meet the objective, particularly Germany, which is often singled out by Trump for criticism.
The EU president said he hoped Trump would remember Europe's strong relationship with the USA as he meets with Russian President Vladimir Putin in Helsinki in the coming days.
Trump left that meeting without signing the joint statement from the seven nations and antagonizing important US allies, including Canadian Prime Minister Justin Trudeau.
While NATO members increased overall defense spending in 2017, for a second consecutive year, the United States was still largely outspending all other members.
"Dear America, appreciate your allies, after all you don't have that many. We pay far too much and they pay far too little", Trump told reporters before leaving the White House on Tuesday morning.
"I'm going to tell NATO: You've got to start paying your bills".
At the time, Johnson was Mayor of London and said Trump was talking "complete and utter nonsense".
Belgian Prime Minister Charles Michel received a letter from the Trump administration earlier this month demanding that the country boost its spending, but Michel said was "not very impressed". He said that money went towards both USA and European security when the United States regards Russian Federation and China as threats.
"I really can't say right now".
At a reception in June celebrating 100 years of American and European partnership in Brussels, Washington's top diplomat to the European Union, Adam Shub, sought to highlight common ground, even on issues such as trade and Iran.
North Atlantic Treaty Organisation released new estimates for allied defence spending in 2018 on Tuesday, showing that Latvia and Lithuania are set to meet the target of spending 2 percent of economic output on the military.
According to an internal European Union memo obtained by Bloomberg and using BEA figures, when including trade in goods, trade in services and primary income from investments, the US has a 12 billion-euro surplus with the EU. "And dear Europe, spend more on your defense because everyone expects an ally that is well prepared and equipped", he added, noting: "Money is important, but genuine solidarity should be more important".
Trump wouldn't say whether he considered Putin a friend or foe, calling him only a "competitor". (Putin did in fact work in the KGB.) "You know what", he said, "Putin's fine".