Earlier in the day, US President Donald Trump held a meeting in Brussels with high-ranking EU officials, including European Council President Donald Tusk, European Commission President Jean-Claude Juncker, European Parliament President Antonio Tajani and EU foreign policy chief Federica Mogherini.
Trump's entourage however warned that the billionaire president will once again press the 28-nation alliance to pay their full share of the alliance's financial burden.
Secretary of State Rex Tillerson said Wednesday that "of course" the United States supports Article 5, though Trump still wants other nations to meet their obligation to spend 2 percent of their gross domestic product on defense.
She is also expected to raise Britain's concerns directly with the president over the leaking to the U.S. press of sensitive details of the investigation into the Manchester attack, including photographs of bloodstained fragments of the bomb.
The president's Brussels meeting with the leaders will be the fourth stop on his inaugural overseas trip.
On his arrival on Wednesday in Brussels, the city he once said had been turned into a "hellhole" by Muslim immigration, the president was greeted by around 9,000 protesters saying "Trump not welcome".
A student-led protest, dubbed "Trump Not Welcome", took place in the Belgian capital yesterday as Trump arrived in the city.
Trump had alarmed the European Union by not only calling on more countries to follow Britain out of the exit door but also by calling it a vehicle for German dominance of the continent. As far the Commission is concerned, President Juncker insisted on intensifying trade cooperation which is a win-win situation for both sides.
The Prime Minister will also back calls by Mr Trump for other allies to take on a greater share of the burden of collective defence - a key demand of the United States president.More news: Sky Investment Group LLC Has $447000 Position in Occidental Petroleum Co. (OXY)
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"But when you see something like what happened a few days ago, you realise how important it is to win this fight".
It remains to be seen if Trump will change his tone about Brussels the way he did about Pope Francis.
At least 9,000 people marched through Brussels on Wednesday night waving blond-haired effigies of the reality TV star president. When Trump tried to lighten the mood with a joke about NATO's gleaming new home base - "I never asked once what the new NATO Headquarters cost" - there was no laughter from his counterparts.
While in Belgium, Trump will unveil a memorial to the terrorist attacks of September 11, 2001, the only time in the alliance's history that the Article 5 mutual defense pledge has been invoked.
However, Trump has since backed away from his comments, and hopes are high in Brussels that he will make a display of solidarity.
U.S president Donald Trump spent a something more than an hour at the European Council "Europa" building.
"NATO's strength allows us to defend our nations at home, to train partners overseas, and to engage in hard but important dialogue", the alliance's secretary general, Jens Stoltenberg, said in a preview of the meeting's agenda on Wednesday. But putting some meat on the pledge, the leaders will agree to prepare action plans by the end of the year, plotting how to reach 2 percent over the next seven years, and show how they will use the money and contribute troops to North Atlantic Treaty Organisation operations.
Trump's wife Melania, meanwhile, is set to visit a museum dedicated to the surrealist artist Rene Magritte and a leading leather store while in Brussels.
Trump is not expected to back the Paris Agreement on climate change in his European Union meeting on Thursday, but the European Union official said: "The fact that we're now in a situation where we can talk about business ... and not existential questions shows the progress we've made".