Officials in Kyiv, however, said they were grateful for vital U.S. help in the war and want to keep working with Washington.

Washington's suspension of military assistance will not lead to a sudden collapse of Ukraine's defensive lines, but it could have a real impact on the war within months, undermining air defences and precision strike capabilities in particular.

The U.S. move won’t have an immediate impact on the battlefield, where Kyiv’s forces are struggling to stem a relentless Russian offensive in eastern Ukraine. Ukrainian forces have slowed Russian advances along the roughly 1,000-kilometer (600-mile) front line, where Russia is slowly gaining ground at a huge human and material cost.