The Trump administration has approved a plan to provide lethal weapons to Ukraine, including Javelin anti-tank missiles, US officials have said.
The move intensifies US support for Ukraine's military as it fights rebels that the United States says are supported, armed and trained by Russia.
The decision is likely to inflame tensions with the Kremlin.
Congress has a chance to stop proposed foreign military sales, but is unlikely to do so in this case.
Politicians have long urged the US to send lethal weapons to Ukraine.
The State Department would not confirm that lethal weapons would be sent, saying only that the US planned to provide "enhanced defensive capabilities".
The State Department called the assistance "entirely defensive".