NATO will offer Ukraine a new headquarters to manage its military assistance at its upcoming 75th anniversary summit in Washington, officials said, an assurance of the alliance’s long-term commitment to the country’s security that has been heralded as a “bridge” to Kyiv’s eventual membership.

President Volodymyr Zelensky of Ukraine — along with some Central European nations — had fervently hoped his country would be offered membership negotiations by NATO at the summit, which runs from July 9 to 11.

Instead, the alliance will announce that it has agreed to set up a mission in Germany to coordinate aid of all kinds to Ukraine over the longer term, American and NATO officials said. The move is intended to send a strong signal of allied commitment, both to Kyiv and to Moscow, which hopes the West will grow tired of supporting the war.

Because the mission will be under NATO’s auspices, it is designed to function even if Donald J. Trump, a sharp critic of the alliance and of aid to Ukraine, wins the U.S. presidency in November.

The Biden administration and NATO officials came up with the idea as a way to give something solid to Kyiv at the summit even as they maintain the time is not right for Ukraine to join.

It is not just that the country is still at war, which could make NATO an active participant in the fighting. President Biden and Chancellor Olaf Scholz of Germany have said that Ukraine must make important reforms to reduce corruption and improve its democracy and rule of law.