BRUSSELS -- President Trump touched down in Brussels Tuesday afternoon ahead of a potentially contentious NATO summit where he's expected to push member nations to spend more on defense.

So far this year, only five out of 29 NATO countries, including the U.S., spent 2 percent of gross domestic product on defense, the NATO standard.

The president has lashed out at countries that fall short. On Tuesday, European Council President Donald Tusk urged Mr. Trump to soften his rhetoric.

US President Donald Trump (R) and US First Lady Melania Trump disembark from Air Force One as they arrive at Melsbroek Air Base in Haachtsesteenweg on July 10, 2018. Benoit Doppagne / AFP/Getty Images

"Dear America, appreciate your allies. After all, you don't have that many. And dear Europe, spend more on your defense," he said.

NATO allies are trying to minimize discord before the president's first summit with Russian President Vladimir Putin next week.

"Frankly Putin might be easiest of them all. Who would think," the president said.

Despite Russia's aggressive moves in Ukraine and confirmed election meddling in the U.S., the president wouldn't say if Putin was friend or foe.

"I really can't say right now. As far as I'm concerned, a competitor, a competitor," he said.

In between the NATO and Putin summits, the president will stop in London. He appeared to take a jab at Prime Minister Theresa May by praising her recently resigned Foreign Secretary Boris Johnson.

"Boris Johnson's a friend of mine. He's been very, very nice to me. Very supportive and I maybe will speak to him when I get over there," the president said.