President Donald Trump reiterated the U.S. commitment to NATO Thursday after a contentious meeting in which he said allies agreed to raise defence spending by US$33 billion amid his reported threats to withdraw.

“Everyone’s agreed to substantially up their commitment,” Trump said in an impromptu press conference in Brussels. “I just want fairness for the United States.”

Trump said he probably could withdraw from NATO without congressional approval, but it won’t be necessary. He said the U.S. commitment to the 29-member North Atlantic Treaty Organization remains very strong and cited “the additional money they will be putting up” and “the level of spirit in that room” during the meetings.

In an unexpected twist, NATO leaders held an unplanned emergency session on the last day of their two-day summit, which has been upended by Trump’s attacks on allies over defence spending. One official said the morning meeting Thursday was taking place against the backdrop of Trump threatening allies to "go it alone" unless if they agree to increase their defence budgets immediately.

Few had expected smooth sailing at the Brussels summit. But the vehemence of Trump’s on-camera breakfast attack on Germany for its backing of a second Nord Stream gas pipeline, and for being “captive” to Russia came as a shock. German Chancellor Angela Merkel on Wednesday gave Trump an uncharacteristically personal response, reminding him that she knew what captive means, having grown up in Soviet-controlled East Germany.

NATO set a goal at a 2014 meeting in Wales for members to spend a set proportion of gross domestic output on defence. But Trump was not content with pressuring NATO allies to raise their defence spending to 2 per cent of economic output and he proposed doubling the target.Trump’s suggestion was informal and made in a closed-door session of on Wednesday, but it did little to ease tensions at an already charged meeting.