Some 90 tonnes of US "lethal aid" has arrived in Ukraine, amid tensions over Russia's troop build-up on the border.
It was the first shipment of a recently approved package of US military aid for Ukraine, and included ammunition for "front-line defenders".
The delivery followed US Secretary of State Antony Blinken's visit to Kyiv this week, where he warned of a tough response if Russia was to invade.
Moscow has denied any plans to attack or invade Ukraine.
US President Joe Biden approved the $200m (£147.5m) security support package in December.
The US embassy in Kyiv said the shipment demonstrated its "firm commitment to Ukraine's sovereign right to self-defense".
"The United States will continue providing such assistance to support Ukraine's Armed Forces in their ongoing effort to defend Ukraine's sovereignty and territorial integrity against Russian aggression," it wrote on Facebook.
Ukrainian Defence Minister Oleksiy Reznikov thanked the US for the aid.
The shipment arrived hours after Russia's foreign minister and his US counterpart held what they called "frank" talks to try to reduce the chance of a wider conflict in Ukraine.
Russia has seized Ukrainian territory before, annexing the Crimean peninsula in 2014. Since then, some 14,000 people have been killed and at least two million fled their homes because of fighting between the Ukrainian army and Russian-backed rebels. A fragile peace agreement was made in 2015.
Now, the head of the military defence alliance Nato has warned there is a real risk of a fresh conflict in Europe after an estimated 100,000 Russian troops amassed on the border.
Moscow has denied it is planning an invasion, but President Vladimir Putin has issued demands to the West which he says concern Russia's security, including that Ukraine be stopped from joining Nato.
He also wants Nato to abandon military exercises and stop sending weapons to eastern Europe, seeing this as a direct threat to Russia's security.
