Ambassador Lincoln Bloomfield (Ret.) on President Trump's public attack against Germany over its gas pipeline deal with Russian Federation.
Speaking after a two-day summit in Brussels, he said allies had committed to spending more than 2% of their annual output (GDP) on defence budgets.
German Chancellor Angela Merkel (L) looks on as US President Donald Trump speaks to other leaders as they pose for a family photo at the Park of the Cinquantenaire during the NATO Summit in Brussels, Belgium July 11, 2018.
"NATO Secretary-General Jens Stoltenberg is sidestepping questions about whether President Donald Trump has asked his allies to double their defence spending ambitions".
Later Mr Trump and German chancellor Angela Merkel had a face-to-face meeting, where the row continued.
Instead, a series of events - a black-tie dinner with business leaders, a meeting with May and an audience with Queen Elizabeth II - will happen outside the bustling city, where Mayor Sadiq Khan has been in a verbal battle with Trump.
Lithuanian President Dalia Grybauskaite told reporters she's comfortable with the Trump-Putin summit, but that it "depends very much on outcomes".
Trump took to Twitter to say publicly what he told North Atlantic Treaty Organisation leaders privately on Wednesday, calling on all allies to meet a commitment agreed in 2014 to spend 2 percent of economic output on defence to counter threats ranging from Russia's military modernisation to militant attacks on European cities.
Mr Trump opened his argument by questioning why Germany pays billions to Moscow for gas supplies but then demands the United States defends it from Russian Federation, saying repeatedly: "I think it's inappropriate".
Merkel, who grew up in communist East Germany, shot back that she knew what it meant to be under Kremlin domination and was glad a united Germany was now able to "make our own independent policies and make our own independent decisions".
On day two, leaders will welcome non-NATO partners including Afghan President Ashraf Ghani and Ukraine's Petro Poroshenko to the alliance's new glass-and-steel headquarters as they seek to focus on policy rather than politics.
NATO Secretary-General Jens Stoltenberg said, "We should first get to two percent", adding that eight of the 29 allies were meeting that target. Trump tweeted an hour before the second day of the summit got underway.
Germany, Europe's biggest economy, spends just 1.24 per cent, compared with 3.5 per cent for the US.
"We made a tremendous amount of progress today", he said.
"We had a fantastic meeting at the end".
The mercurial tycoon said before leaving Washington that his meeting in Helsinki with Russian President Vladimir Putin on Monday "may be the easiest" part of his European tour, which also includes a trip to Britain, where the government is in crisis over Brexit.
"The president came out of Singapore saying he'd ended the nuclear threat from North Korea", said one USA official, referring to Trump's meeting with North Korean leader Kim Jong Un.