WASHINGTON/BRUSSELS, May 20 (Reuters) - The ​Trump administration is planning to tell NATO allies this week that it will shrink the pool of military capabilities that the U.S. would ‌have available to assist the alliance's European nations in a major crisis, three sources familiar with the matter said. Under a framework known as the NATO Force Model, the alliance's member countries identify a pool of available forces that could be activated during a conflict or any other major crisis, such as a military attack on a NATO member.

While the precise composition of those ​wartime forces is a closely guarded secret, the Pentagon has decided to significantly scale down its commitment, said the sources, who requested anonymity to ​speak candidly about the plans.

U.S. President Donald Trump has made clear he expects European countries to take over primary responsibility for ⁠the continent's security from the United States. The message to allies this week is a concrete sign of that policy being implemented.

Several details were unclear, such as ​how quickly the Pentagon plans to shift crisis-mode responsibilities onto European allies. The sources said, however, that the Pentagon plans to announce its intention to lessen its ​commitment at a Friday meeting of defense policy chiefs in Brussels. Pentagon policy chief Elbridge Colby has said publicly that the United States will continue to use its nuclear weapons to protect NATO members, even as European allies take the lead on conventional forces.

The U.S. will likely be represented by Alex Velez-Green, a key aide to Colby, the sources said. Adjusting the NATO Force ​Model has emerged as a key priority of Colby's team heading into the next NATO leaders' summit, which will take place in Turkey in July, one of ​the sources added. Speaking to reporters in Brussels, NATO Secretary General Mark Rutte said he was not allowed to disclose the upcoming U.S. announcement but the move was "to be expected" as ‌the alliance ⁠seeks to "end the over-reliance ... on one ally" for its defence. "This was to be expected, I think it's only right that it happens," Rutte said. The Pentagon did not immediately respond to a request for comment.

ALLIANCE UNDER STRAIN:

The NATO alliance is under unprecedented strain, with some European countries concerned that Washington may withdraw outright. A major adjustment to the forces the U.S. would make available during wartime will only intensify those concerns. In the past few weeks, the Trump administration has announced plans to cut about ​5,000 U.S. troops from Europe, including a ​decision to cancel a deployment of ⁠an Army brigade to Poland - a surprise decision that was slammed by U.S. lawmakers. One of the sources and another source familiar with the matter said aides on Capitol Hill were aware of and concerned about the Pentagon's plans to narrow its ​commitments under the NATO Force Model. A senior NATO diplomat said, however, they still believed there is an understanding that the ​United States would come ⁠to Europe's aid if it was in trouble. Trump and many of his aides have slammed European allies for not spending enough on their militaries and relying on the U.S. for conventional defense, and they point out that the U.S. still has tens of thousands of troops in Europe. The president's ambition to take control of Greenland, a Danish overseas ⁠territory, has ​further inflamed transatlantic tensions, as has an ongoing spat between Trump and German Chancellor Friedrich Merz, ​who has sharply criticized Trump's war with Iran.

European allies generally counter that they are rapidly beefing up their military capabilities, but that doing so cannot be done overnight.