Secretary of State Rex Tillerson ratcheted up pressure on NATO allies Friday to increase their defense spending, despite pushback from Germany’s top diplomat over President Trump’s determination to make members of the Western military alliance boost their military budgets.

Addressing a meeting of NATO’s 28 foreign ministers, Tillerson said he wanted alliance leaders to agree at a May summit to come up with concrete plans by the end of the year to meet budget guidelines. Friday’s conference — hastily moved up after Tillerson initially announced he would skip it so he could attend meetings between Trump and China’s leader next week — was held amid concerns about the U.S. commitment to NATO following Trump’s calls to increase spending.

“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,” Tillerson told the foreign ministers. “Allies must increase defense spending.”

The effort has met with resistance from German Foreign Minister Sigmar Gabriel, who said the push from Washington was unrealistic and based on a mistaken interpretation of the spending targets, which are not binding. Germany is NATO’s largest economy after the United States, but it lags far behind in its defense spending. Twisting Berlin’s arm to increase its military expenditures is key to Trump’s effort to shift more of the burden for Europe’s defense to Washington’s NATO partners.

“More money doesn’t mean more security,” said Gabriel, who is a member of Germany’s center-left Social Democratic Party and has long been skeptical of defense spending increases. He said that meeting NATO spending guidelines would require Germany to pour an additional $37 billion a year into its defense budget, which he said was “totally unrealistic.” NATO leaders have pledged to increase annual defense outlays to 2 percent of their gross domestic products by 2024, but those shares are targets rather than requirements.

After Trump met with German Chancellor Angela Merkel in mid-March, he wrote on Twitter that “Germany owes vast sums of money to NATO & the United States must be paid more for the powerful, and very expensive, defense it provides to Germany!”

That approach misstated the way NATO works, since allies contribute military capabilities rather than actual money to the alliance.

Tillerson’s trip built on an ongoing pattern from the Trump administration: The president says something at odds with U.S. policy or existing commitments, then leaves it to subordinates to reassure allies. Defense Secretary Jim Mattis and Vice President Pence engaged in the same exercise during February trips to Europe.

One of the first tests of the tug-of-war will be at the NATO summit May 25, when Trump will sit down with a wide array of allied leaders for the first time. Allies will see whether his personal message is consistent with the basic bargains worked out by U.S. officials ahead of time, or whether he will make different, tougher demands as leaders hammer out their positions.

The inconsistencies are not confined to the United States: Merkel and her center-right Christian Democratic Union allies, who rule in a coalition with Gabriel’s center-left party, have been more willing than Gabriel to entertain Trump’s requests. Defense increases are broadly unpopular in Germany ahead of September elections, and some of Gabriel’s aggressive push was for domestic electoral consumption. But he rejected out of hand Tillerson’s proposal that nations create spending plans with specific, year-by-year targets, setting up a potential showdown at the summit.

“We need many different tools to stabilize our neighborhood,” NATO Secretary General Jens Stoltenberg said after the meeting, countering the German position that development aid should also be counted toward security spending. “It’s not either development or security, it’s development and security.”

NATO diplomats said the closed-door meetings with Tillerson were cordial. But some quietly criticized him for spending less than five hours on the ground in Brussels and returning to Washington before the conference concluded. Previous secretaries of state have often held news conferences at the end of such conclaves; Tillerson took no questions in Brussels.

Many diplomats counted it as a victory to have persuaded Tillerson to come in the first place. The gathering was initially scheduled for next week, overlapping with Chinese President Xi Jinping’s visit to Florida to meet Trump. Tillerson planned to skip the NATO event, meaning that he would have met with Chinese and Russian leaders before he met with his alliance counterparts.

“Going to Moscow before seeing members of the alliance did not go down well,” a senior NATO diplomat said ahead of the meeting, speaking on condition of anonymity to talk frankly about allied perceptions of the Trump administration.

NATO allies have been concerned about Trump’s approach to Russia, one that prioritizes cooperation with the Kremlin to defeat the Islamic State.

But Tillerson took a hard line against Russia during the meeting, saying that the United States would maintain its refusal to recognize Moscow’s 2014 annexation of Ukraine’s Crimean Peninsula and would keep deploying troops to Eastern Europe as a deterrent.

NATO is “fundamental to countering both nonviolent, but at times violent, Russian agitation and Russian aggression,” Tillerson told his fellow foreign ministers.

The message was reassuring to NATO nations that border Russia, which have felt especially vulnerable since tensions increased in 2014.

“Those were things that my part of the world was very happy to hear,” said Latvian Foreign Minister Edgars Rinkevics, who also met with Tillerson in Washington earlier this week alongside his counterparts from the other Baltic nations of Lithuania and Estonia.

Tillerson repeated U.S. desires for NATO to do more to fight terrorism, but he offered no concrete requests and said the Trump administration was still working on specific roles in which NATO could contribute. Although NATO has increased training for Iraqi troops and offered its AWACS surveillance planes to help efforts to defeat the Islamic State, many alliance nations remain skeptical about how well NATO’s structures are suited for counterterrorism.

Skeptical diplomats say the intelligence agencies and police forces of individual NATO nations are better-adapted to fighting domestic terrorism threats. Direct intervention on the ground in Syria and Iraq may be most effective if countries offer their militaries on an individual basis rather than in a joint effort through NATO structures, they say.

The NATO meeting came after a one-day trip to Ankara, where Tillerson met with Turkish President Recep Tayyip Erdogan and his top deputies in a bid to build ties despite tensions over clashing approaches to fighting the Islamic State in Syria. It was Tillerson’s second trip to Europe since he hopped to the State Department from his previous position as chief executive of ExxonMobil.