BRUSSELS — President Donald Trump continued to hammer America’s NATO allies over their defense spending for a second day on Thursday in Brussels. In a series of tweets from the Belgian capital, Mr. 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 members of the military alliance meet their pledge to spend 2 percent of GDP on defense, which “must ultimately go to 4%!”

Sources familiar with the first round of closed-door talks on Thursday told the Reuters news agency that Mr. Trump carried his firm stance on military spending from Twitter into the meeting rooms. According to one Reuters source, Mr. Trump, “forcibly restated his position on wanting NATO members to reach 2 percent spending target to a short a deadline.”

The Reuters sources said Mr. Trump did not go as far as to threaten to pull the United States out of the transatlantic defense alliance — as some European organizations reported — but officials told the Associated Press that his demands for increased national defense spending were enough to prompt NATO officials to declare an emergency session in Brussels on Thursday.

The U.S. president has taken an aggressive tone during the summit, questioning the value of an alliance that has defined decades of American foreign policy, torching an ally and calling for a massive increase in European defense spending. White House officials announced that Mr. Trump would give an unscheduled news conference on Thursday in Brussels amid the tumult over his demands for increased spending.

CBS News will air the president’s remarks in a Special Report from approximately 1:15 a.m. Eastern, which will also be streamed live on CBSN

Under fire for his warm embrace of Russia’s Vladimir Putin, Mr. 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 that 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 will include talks with the leaders of Azerbaijan, Romania, Ukraine and Georgia. In the afternoon, he heads to his next stop: the United Kingdom.

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

With scorching language, Mr. 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 a 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 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 a decades-old alliance launched to counterbalance Soviet aggressions. And it comes just days before Mr. Trump sits down with Putin at the conclusion of his closely watched European trip.

Mr. 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.

Amid the tumult, British Prime Minister Theresa May sounded a call for solidarity among the allies, saying, “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. And, as long as Russia persists in its efforts to undermine our interests and values, we must continue to deter and counter them.”

From Brussels, Mr. Trump heads to England, where May’s government is in turmoil over her plans for exiting the European Union.

Although administration officials point to the long-standing alliance between the United States and the United Kingdom, Mr. Trump’s itinerary 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 Mr. Trump.

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