Ukraine and some of its strongest supporters are pushing NATO to formally invite Kyiv to become a member of the military alliance during a major upcoming summit, but their efforts are facing significant behind-the-scenes pushback from the United States and Germany, according to a dozen current and former officials familiar with the matter.

Kyiv has backing from Eastern European countries already in the military alliance, including Poland and the Baltic states, which view extending NATO membership to Ukraine as the most effective and least costly way to blunt Russia’s irredentist ambitions in Eastern Europe. Yet other Western NATO officials, mostly notably in Washington and Berlin, believe that it’s too soon to kick-start the process of admitting Ukraine to NATO while the country is still fighting a war against Russia.

Both the United States and Germany are top donors of military and economic aid to Ukraine. Officials in both countries insist that Ukraine should join NATO eventually, but that now isn’t the time to start the process. They said they believe the immediate focus should be on continuing to supply Ukraine with weapons and munitions to keep up the fight against Russia in the near term.