US president has been questioning the value of NATO and has called on member states to increase military spending.

Leaders from countries in the North Atlantic Treaty Organization (NATO) held an unplanned session on Thursday as US President Donald Trump ramped up his demand for member countries to increase their contribution to the alliance.

Trump reportedly singled out Germany, Spain and Belgium “to reach two percent spending target” within a short deadline, two sources told Reuters news agency.

The DPA news agency earlier reported that during the meeting, Trump threatened to pull the US out of NATO, but the report was denied by the two Reuters sources.

Following the meeting, Trump said that “tremendous progress has been made” with more countries increasing their commitment to NATO.

“People are paying money than they’ve never paid before,” Trump said, but failed to provide details on when allied countries would increase their commitment.

Al Jazeera’s diplomatic editor, James Bays, described the unplanned meeting on Thursday as “explosive”, adding that it disrupted a scheduled NATO discussion on Ukraine and Georgia.

“The NATO leaders weren’t really expecting this.”

German Chancellor Angela Merkel acknowledged that there was a disagreement during the meeting, but tried to play down its significance, our correspondent said.

Earlier on Thursday, the US president posted a series of messages on social media, hammering US allies during the second day of meetings with leaders of the military alliance.

Trump, in a series of tweets from Brussels, said that, “Presidents have been trying unsuccessfully for years to get Germany and other rich NATO Nations to pay more towards their protection from Russia.”

He complained that the US “pays tens of Billions of Dollars too much to subsidize Europe” and demanded that member nations meet their pledge to spend 2 percent of GDP on defense, which “must ultimately go to 4%!”

The two percent of gross domestic product spending target is originally scheduled for 2024.

Under fire for his warm embrace of Russia’s Vladimir Putin, Trump on Wednesday turned a harsh spotlight on Germany’s own ties to Russia, alleging that a natural gas pipeline venture with Moscow has left Angela Merkel’s government “totally controlled” and “captive” to Russia.

He continued the attack Thursday, complaining that, “Germany just started paying Russia, the country they want protection from, Billions of Dollars for their Energy needs coming out of a new pipeline from Russia.”

“Not acceptable!” he railed before arriving late at NATO headquarters for a morning of meetings that included talks with the leaders of Azerbaijan, Romania, Ukraine and, Georgia.

Peter Navarro, director of the White House National Trade Council, echoed Trump’s rhetoric, telling Fox Business Network that “Germany is a tremendous problem, both for Europe itself, and for the United States in this sense.”

Trump said the current situation is not fair to the taxpayers in the US [AP]

“It’s really extraordinary that Donald Trump has to be the person to point out that the emperor in Europe has no clothes,” he said.

Tough rhetoric

The tough rhetoric against a core ally comes just days before Trump is set to meet one-on-one with Putin in Finland.

With scorching language, Trump questioned the necessity of the alliance that formed a bulwark against Soviet aggression, tweeting after a day of contentious meetings: “What good is NATO if Germany is paying Russia billions of dollars for gas and energy?”

During the meetings, he demanded via tweet that NATO countries “Must pay 2% of GDP IMMEDIATELY, not by 2025” and then rattled them further by privately suggesting member nations should spend four percent of their gross domestic product on defence – a bigger share than even the United States currently pays, according to NATO statistics.

It was the most recent in a series of demands and insults that critics fear will undermine a decades-old alliance launched to counterbalance Soviet aggression. And it comes just days before Trump sits down with Putin at the conclusion of his closely watched European trip.

Next stop: England

Trump has spent weeks berating members of the alliance for failing to spend enough of their money on defence, accusing Europe of freeloading off the US and raising doubts about whether he would come to members’ defence if they were ever attacked.

He described the current situation as “disproportionate and not fair to the taxpayers of the United States”.

However, a formal summit declaration issued by the NATO leaders Wednesday reaffirmed their “unwavering commitment” to the two percent pledge set in 2014 and made no reference to any effort to get to 4 percent.

Merkel, who grew up in communist East Germany, shot back that she had “experienced myself a part of Germany controlled by the Soviet Union, and I’m very happy today that we are united in freedom as the Federal Republic of Germany and can thus say that we can determine our own policies and make our own decisions and that’s very good”.

From Brussels, Trump heads to England on Thursday after, where he is expected to be greeted by a massive protest against his policies.

“I think it’s fine. I think they like me a lot in the UK,” Trump said dismissing the expected protests.