NATO leaders entered a special emergency session on Thursday after President Donald Trump was said to have spoken very bluntly about his demands that the countries spend more on defense.

During the summit, Trump broke diplomatic protocol by calling German Chancellor Angela Merkel by her first name, saying, "Angela, you need to do something about this," a source told Reuters.

Trump has repeatedly slammed Merkel for supporting a new pipeline that would cement Berlin's client relationship with Russia and increase Moscow's influence.

Trump also demanded NATO countries up their defense spending immediately and said all allies should eventually double defense spending to a wartime-like 4% of gross domestic product.

NATO leaders entered a special emergency session on Thursday after President Donald Trump was said to have spoken very bluntly about his demands that the countries spend more on defense.

During the summit, Trump broke diplomatic protocol by calling German Chancellor Angela Merkel by her first name, saying, "Angela, you need to do something about this," a source told Reuters.

Leaders of Afghanistan and Georgia, non-NATO members, were asked to leave for the emergency session.

Trump singled out Germany on Wednesday when he accused the country of being "totally" controlled by Russia because Russia provides a large share of its oil and natural gas. Merkel fired back that Germany was independent and a strong NATO ally.

"The language was much tougher today," a source told Reuters. "His harshest words were directed at Germany, including by calling her Angela — 'You, Angela.'"

Trump emerged from the session to make an unscheduled statement where he said that he had communicated to other NATO countries he would be "extremely unhappy" if they didn't quickly up their spending but that they had agreed to do so.

"We had a very intense summit," Merkel told reporters after the session, per Reuters.

Trump's NATO grudge

First row, from left: Merkel, Belgian Prime Minister Charles Michel, NATO Secretary General Jens Stoltenberg, Trump, and British Prime Minister Theresa May at the NATO summit. Sean Gallup/Getty Images

Trump and other US presidents before him have pressed European leaders to spend more on defense to contribute to NATO, but Trump has consistently advocated an accelerated timeline.

NATO countries agreed to each spend 2% of their gross domestic product on defense by 2024, but so far only a handful meet that mark. Germany, Europe's richest country, spends 1.24% of its GDP on defense, and it's an unpopular topic there.

Not only did Trump demand on Twitter on Thursday that countries meet the 2% level by this year, not 2024, but he also said they should eventually hit 4%, which is more than even the US currently spends. Spending 4% of GDP on defense would represent nearly wartime levels of investment.

Trump has repeatedly slammed Merkel for supporting a new pipeline that would cement Berlin's client relationship with Russia and increase Moscow's influence. Energy exports represent Russia's main source of revenue, and Trump argues that the pipeline undermines NATO's purpose, as it's designed to counter Russian aggression.