BRUSSELS (AP) -- President Donald Trump's demands that NATO member nations dramatically increase their spending on defense led leaders of the military alliance to meet in an emergency session on Thursday, officials said.
Two officials attending the summit confirmed the session. The officials spoke on condition of anonymity to describe the development.
Trump is expected to hold a news conference Thursday.
Trump on Thursday stepped up his pressure tactics on fellow NATO nations, calling out U.S. allies on Twitter as he attended a second day of meetings with leaders of the military alliance.
In a series of tweets from Brussels, Trump said "Presidents have been trying unsuccessfully for years to get Germany and other rich NATO Nations to pay more toward their protection from Russia."
He complained the United States "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%!"
Trump has taken an aggressive tone during the NATO summit, questioning the value of an alliance that has defined decades of American foreign policy, torching an ally and proposing a massive increase in European defense spending.
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 morning meetings that will include talks with the leaders of Azerbaijan, Romania, Ukraine and Georgia. In the afternoon, he heads to his next stop: the United Kingdom.
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."
"What's more surprising, the fact that the President Trump is calling them out on that or that previous presidents haven't?" he asked. "It's really extraordinary that Donald Trump has to be the person to point out that the emperor in Europe has no clothes."
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."
During the trip, Trump has 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?"
He demanded that NATO countries "Must pay 2% of GDP IMMEDIATELY, not by 2025" and then rattled them further by privately suggesting member nations should spend 4 percent of their gross domestic product on defense — 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 the decades-old alliance, coming days before Trump sits down with Putin at the conclusion of his closely watched European trip.
Lithuanian President Dalia Grybauskaite told reporters she's comfortable with the Trump-Putin summit, but that it "depends very much on outcomes."
"These two people are very different, very interestingly different. Hopefully nothing bad happens," she said.
Trump has spent weeks berating members of the alliance for failing to spend enough of their money on defense, accusing Europe of freeloading off the U.S. and raising doubts about whether he would come to members' defense 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 2 percent pledge set in 2014 and made no reference to any effort to get to 4 percent.
But Trump has been more conciliatory behind the scenes, including at a leaders' dinner Wednesday.
"I have to tell you that the atmosphere last night at dinner was very open, was very constructive and it was very positive," Kolinda Grabar-Kitarovic, the president of Croatia, told reporters.
Amid the tumult, British Prime Minister Theresa May, whose government is in turmoil over her plans for exiting the European Union, sounded a call for solidarity among allies.
"As we engage Russia we must do so from a position of unity and strength - holding out hope for a better future, but also clear and unwavering on where Russia needs to change its behavior for this to become a reality," she said.
Although Trump administration officials point to the long-standing alliance between the United States and the United Kingdom, Trump's itinerary in England will largely keep him out of central London, where significant protests are expected.
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.
Woody Johnson, the U.S. ambassador to the United Kingdom, dismissed the significance of the protests, telling Fox News that one of the reasons the two countries are so close "is because we have the freedoms that we've all fought for. And one of the freedoms we have is freedom of speech and the freedom to express your views. And I know that's valued very highly over here and people can disagree strongly and still go out to dinner."
He also said meeting the queen would be an experience Trump "will really cherish."