US Secretary of State Antony Blinken tells President Volodymyr Zelensky that military aid is "now on its way" during a surprise visit to Kyiv.
The top US diplomat tells the Ukrainian president the weapons will "make a real difference" against the ongoing Russian aggression on the battlefield.
A senior US official says the American-funded weapons now arriving at the front line include air defence interceptors, artillery and ATACMS long-range precision guided missiles.
The visit comes as the country struggles to hold back a major Russian incursion near its second biggest city of Kharkiv.
The cross-border offensive in Kharkiv region is one of the most significant Russian ground attacks since the start of the war.
Earlier UK Defence Secretary Grant Shapps told the BBC he thinks the world "took its eye off the ball" regarding Ukraine's need for arms.