President Trump does not support NATO membership for Ukraine, Pete Hegseth, the U.S. defense secretary, said on Wednesday, in a significant blow to the country’s hopes of eventually joining the mutual-defense alliance.

Ukraine has made membership in NATO a key strategic aim, based on the idea that the alliance’s blanket of protection would guard against future Russian aggression.

But even before this week’s events, membership in NATO was a distant dream for Ukraine. NATO countries have supported Ukraine with military aid, but have shown little appetite for taking on a new member that could potentially draw them into a direct war with Russia.

In his second term, Mr. Trump — who has questioned NATO’s relevance and even threatened to withdraw from the alliance — now appears to be moving even further from the idea that Ukraine could join the group in the future.