"Dear America, appreciate your allies, after all you don't have that many", Tusk said, before reminding Trump that European troops had fought alongside Americans in Afghanistan following the September 11, 2001 attacks on the United States.
She did not directly answer a question about Trump's friendship with Johnson, her former foreign secretary who quit on Monday in protest at her plan for Brexit. "I hope you have no doubt this is an investment in our security, which can not be said with confidence about Russian & Chinese spending".
"Those are stupid people", Trump said, according to the report; "you shouldn't listen to them".
Trump was asked about the UK and replied: "The UK, that's a situation that's been going on for a long time".
NATO's European allies have spent more on defense for a second consecutive year in 2017 but the majority of them still failed to meet the target of spending 2 percent of their gross domestic product on defense.
The NATO alliance has been critical to the USA since its formation in 1949 when it was created in response to the Soviet Union.
The meeting will be closely watched to see whether Trump will rebuke or embrace Putin, who has repeatedly denied the allegations of election meddling, in spite of evidence to the contrary.
The European Union is the top export market for the United States at $501 billion (£377.5 billion) in goods and services in 2016, the latest available figures provided by the Office of the U.S. Trade Representative.
Many US presidents have urged European governments to spend more on their militaries.
North Atlantic Treaty Organisation countries have set a goal of spending 2 percent of their GDP on the military, but only a handful of countries have actually met that target.
In this context, Tusk and others seem concerned Putin will take advantage of the fact that Trump is not standing by key U.S. allies. In March 2016 he said it was "obsolete and disproportionately too expensive" and on Monday, ahead of the summit, he suggested (on Twitter, of course), that "by some accounts, the U.S. is paying for 90% of Nato" and on Tuesday raged: "NATO countries must pay MORE, the United States must pay LESS. Charge us big Tariffs (& Barriers)!" "[Trump] is going to talk to Russian Federation, to Putin about his malign influence in so many areas", she said.
George Clooney injured in scooter accident in Italy
The Hollywood actor collided with a Mercedes auto on Tuesday morning while in Puntaldia, Sardinia, on the way to a TV set. George Clooney is interviewed at the premiere for "Suburbicon" in Los Angeles, California last October.
Mr Trump has been accused of not really understanding the funding.
PSG make world record bid for former Liverpool ace, Coutinho
The pair clashed last season over penalty taking duties and it is believed that the 26-year-old would rather see Cavani leave. PSG remain at the centre of an investigation by UEFA after they spent nearly £400m on Neymar and Kylian Mbappe last summer.
New Zealand cyclist wins stage on Tour de France
They stopped the clock on 38:55 but they were soon out of the "hotseat" as Sky - second off - came through five seconds quicker. As I've said from the start , the legs are good. "It just shows that we are here, too, and let's get it on".
Real Madrid's Ronaldo signs for Juventus
Cristiano Ronaldo has agreed to move to Juventus after nine years at Real Madrid , the La Liga side have confirmed. In an open letter, Ronaldo thanked the Madrid club and its fans for " nine wonderful years ." .
Queen marks 100 years of RAF ahead of flypast
The Queen looked well after skipping Prince Louis's Christening on Monday, following a busy week in Scotland last week. Kate Middleton and Prince William at Prince Louis' christening service.
Prince Louis' Godparents Revealed Ahead of Today's Christening
After Meghan Markle confirmed her father Thomas won't attend the wedding, Prince Charles offered to walk the bride down the aisle. Meghan was christened and confirmed in the oak-panelled Chapel Royal by the archbishop, who also married her and Harry in May.