During the summit, Trump reportedly broke diplomatic protocol by calling German Chancellor Angela Merkel by her first name, saying "Angela, you need to do something about this", a source told Reuters.
The summit in Brussels is shaping up as the alliance's most hard in years, against a backdrop of deepening transatlantic tensions in fields ranging from trade to energy and defence.
Trump wants the spending to go up to 4% of GDP, which is more than even the USA now spends on defense.
But the President's mid-air tweets aboard Air Force One slamming North Atlantic Treaty Organisation allies for failing to meet their defense spending commitments and upbraiding the European Union over what Trump perceives as an unfair trading relationship came as little surprise to the European counterparts Trump is set to meet with over the next week. Because of me, they've raised about $40bn over the past year.
But with tensions in the Western alliance smouldering over Trump's trade tariffs on European steel and his demands for more contributions to ease the burden on USA taxpayers, his earlier remarks fuelled concerns among allies for the United States role in keeping the peace that has reigned since World War II.
"NATO is not a stock exchange where you can buy security".
Trump stressed that NATO's budget had been unfair to the United States but now he was sure it would be fair.
Trump arrived on the back of a barrage of criticism of Europe on issues ranging from trade to energy and above all his claims that the continent freeloads on the back of America for its defence.
U.S. tech firms may feel brunt of China's trade counter attack
The initial USA tariff list focused on Chinese industrial products in an attempt to limit the impact on American consumers. Beijing did not retaliate when the U.S. placed tariffs on steel and aluminum products.
Trump last week told senior aides he was going to make threats about defense spending and that he was determined to flip the table over before he left, a senior administration official said ahead of Thursday's drama, speaking on condition of anonymity to discuss sensitive summit planning.
Hannity pointed out that Germany is spending billions of dollars on an energy pipeline between them and Russian Federation, and that the country's former chancellor, Gerhard Schröder, is the one leading the business effort. Citing the alliance's Secretary-General Jens Stoltenberg, he said: "Secretary Stoltenberg gives us total credit, meaning me, I guess, in this case, total credit".
He said the United Kingdom contributes $61 billion, Germany $51 billion and Canada "an embarrassing" $21 billion. But at the bilateral meeting he exulted: "We have a very, very good relationship with the Chancellor".
"It will be over a relatively short period of years", Trump said.
Germany, Europe's biggest economy, spends just 1.24 per cent, compared with 3.5 per cent for the US.
There had been concern that President Trump might leave the alliance. We are good partners, and we wish to continue to cooperate in the future.
Trump has described the spending situation as "disproportionate and not fair to the taxpayers of the United States". "I don't know why people would believe them when they say their description of the meeting".
- 'They make good love in Croatia!' - Lovren explains incredible talent pool
- Meghan Markle high-fives children at Croke Park during visit
- Trump says added North Atlantic Treaty Organisation funds not enough for U.S.
- Merkel responds to Trump: 'I have witnessed' Germany under Soviet control
- Cornwall woman takes vehicle for accidental, two-week joyride
- Trump brands Germany a "captive of Russian Federation ", days before he meets Putin
- Luka Modric slams English media following Croatia win
- Liverpool willing to smash transfer record to sign Paulo Dybala
- Judge Says US Must Reunite Migrant Families Or Face Penalties
- Turkey's Erdogan assumes presidency, unveils cabinet