According to a report in The Wall Street Journal, the US modified its rockets before sending them to the Ukrainians.
The US Army Tactical Missile System rockets, which have a range of nearly 200 miles, can't be fired because of the changes.
The US was worried that it could acquire long-range projectiles from other countries if it didn't give Ukraine any.
Rather than using long-range weapons that can hit Russian forces in Ukrainian territory, the launchers use shorter-range weapons that can hit Russia itself.
The US doesn't think that the Ukrainians need every possible tool to repel the invasion, so it's not giving them weapons that it thinks could lead to a war.
Maria Zakharova, spokeswoman for the Russian Foreign Ministry, said in September that the US would become a direct party to the conflict if it supplied missiles to Kyiv.
The US has supplied the Ukrainians with at least 20 HIMARS. It was reported in September that a new weapons package would double the number of weapons in the country.
The high-precision rocket launchers have been a crucial addition to Ukraine's arsenal, allowing it to strike distant Russian positions and then move out of the way quickly to avoid reprisal.
According to a defense official, Russia has failed to destroy a single weapon since the beginning of the conflict.
According to the Journal, rockets have been used to strike Russian depots and command centers. In Russia, the weapons are fearsome.
According to the publication, the Pentagon, White House and Ukrainian military wouldn't comment on the changes.
Lawmakers from both sides of the aisle have been pushing the Biden administration to provide Ukraine with a fleet of advanced drones that would greatly increase the lethality of the country's armed forces. The drones are capable of carrying four Hellfire missiles.
According to an earlier report by The Wall Street Journal, the Biden administration is reluctant to give the weapons due to fears that they could end up in the hands of US adversaries.