In his first meeting with NATO, Secretary of State Rex Tillerson on Friday delivered a firm rebuke of Russian “aggression” and promised allies that the United States will stand by their side -- while also demanding they spend more on defense and do more to fight terrorism.

Tillerson participated in abbreviated discussions with foreign ministers from the 27 other NATO member nations, who were sent scrambling last week to accommodate the top U.S. diplomat after he said he could not attend the meeting originally planned for early April.

What was supposed to be a two-day meeting was compressed into a half-day of talks; Tillerson spent just under five hours at NATO headquarters in Brussels and left for Washington before NATO Secretary-General Jens Stoltenberg gave a closing press conference.

The foreign ministers’ meeting is crucial because it lays the groundwork for a NATO summit with heads of state in May, which will be President Trump’s first overseas trip since taking office. Trump has had to work hard to dispel doubts he created about his commitment to NATO, an alliance that has served as the foundation of Western security since World War II.

“The United States is committed to ensuring NATO has the capabilities to support our collective defense. We understand that a threat against one of us is a threat against all of us,” Tillerson said.

But, he added, “as President Trump has made clear, it is no longer sustainable for the U.S. to maintain a disproportionate share of NATO’s defense expenditures.”

The United States is amping up pressure on NATO members to increase their defense spending to 2% of gross domestic product, in line with a 2014 agreement among the alliance’s member countries to meet the target by 2024.

Only five NATO countries meet the 2% threshold. The U.S. spends 3.61% of its GDP on defense, more than any other member of the alliance.

Tillerson said that if countries have not met the 2% spending goal by the end of the year, they should at least have a concrete plan “that clearly articulates how, with annual milestone progress commitments, the pledge will be fulfilled.”

Pressure to meet that strict deadline is likely to upset some allies.

German Vice Chancellor Sigmar Gabriel told reporters before Friday’s meeting that he thinks it would be “completely unrealistic” for Germany to bring its military defense spending up to 2% of GDP.

“I don’t know any politician in Germany who thinks that this would be reachable or desirable,” Gabriel said.

The allies’ agreement to boost spending is a loose set of guidelines, but is not binding, Gabriel argued.

Germany is increasing its military spending this year to $39 billion, or 1.2% of its GDP. Gabriel rejected the Trump administration’s focus on military expenditures, arguing that humanitarian aid and Germany’s spending to take in refugees should be considered part of the defense budget.

Other foreign ministers were more accepting of the Trump administration’s pressure to step up defense spending. Several countries have signaled they will start spending 2% of GDP before the 2024 deadline. Romania says it will reach 2% this year.

In private talks with the ministers, Tillerson harshly criticized Russian “aggression” and “intimidation” in Ukraine and pointedly blamed Russia for a recent escalation in violence there. He said, twice, that U.S. sanctions slapped on Moscow as punishment would remain in place. This was significant because in his previous career as CEO of ExxonMobil, Tillerson had lobbied against the sanctions, which he said were causing the firm to lose money on exploration deals in Russia.

Several ministers from Eastern European countries said they were reassured by Tillerson’s rebuke of Russia. There have been concerns that Trump would seek closer ties to Russian President Vladimir Putin. Baltic countries Estonia, Lithuania and Latvia see Russia’s intervention in Ukraine as a particular threat.

Tillerson “has no illusions about Russia,” Latvian Foreign Minister Edgars Rinkevics said in an interview.

Estonian Foreign Minister Sven Mikser also seemed satisfied. “When it comes to security and defense, I think we are going to see much more continuity than perhaps we expected,” Mikser said. “I think we may see greater or a bigger departure from [former President] Obama’s policies in other areas” such as trade and environmental policy.

Tillerson also called on allies to take a greater role in the fight against terrorism.

“NATO can and should do more,” he said. “Fighting terrorism is the top national security priority for the United States, as it should be for all of us.”

Ministers said they wanted to hear specific proposals for the fight on terrorism.

“There is a willingness to involve NATO more,” Rinkevics said. “What would be very good is that we also use those two months [until the May meeting] not only talking about defense spending, which is very important, but also preparing more concrete decisions.”

Tillerson’s earlier announcement that he would skip the ministerial meeting struck a nerve among the alliance members, coming at a sensitive time when tensions between the Trump administration and NATO allies have soared.

The schedule change caused an awkward protocol shuffle, with a handful of foreign ministers unable to make it to Brussels.

British Foreign Minister Boris Johnson described Tillerson’s message as the need to adapt.

“The reason [NATO] is the most successful alliance in history is because it’s continually been able to adapt,” Johnson said. “Obviously we face new challenges now from Russia, a resurgent Russia getting up to all kinds of mischief in the Western Balkans and elsewhere…. The crucial message … is that as NATO adapts and faces new challenges, we have to fund this organization properly.”

Stoltenberg praised what he described as “transatlantic unity. “The meeting today was important because it was one meeting in a series of meetings where the new U.S. administration has come to Europe and expressed strong commitment to NATO, to the transatlantic bond,” Stoltenberg said.

“I believe it’s very important to remember that fair burden-sharing or increased defense spending across Europe and Canada is not just something you have to do to please the United States,” he added. “It is about investing in our own security, in the security of Europe, because Europe is very close to the turmoil, the violence in North Africa, Iraq, Syria, and we’re closer also to Russia.”

Stupp is a special correspondent. Times staff writer Wilkinson reported from Washington.