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 defenses and precision strike capabilities in particular.

U.S. deliveries of military aid via Ukraine's western border halted at 3:30 a.m., a source with knowledge of the matter said, after U.S. President Donald Trump paused assistance late on Monday following an extraordinary clash with Ukrainian President Volodymyr Zelenskyy last week.

The move suspends any further drawdown from $3.85 billion of military aid approved by Congress that Trump inherited from his predecessor, Joe Biden. It also appears to halt deliveries of military equipment already approved by Biden.