WASHINGTON (AP) — Ukraine has sidelined U.S.-provided Abrams M1A1 battle tanks for now in its fight against Russia, in part because Russian drone warfare has made it too difficult for them to operate without detection or coming under attack, two U.S. military officials told The Associated Press.

The U.S. agreed to send 31 Abrams to Ukraine in January 2023 after an aggressive monthslong campaign by Kyiv arguing that the tanks, which cost about $10 million apiece, were vital to its ability to breach Russian lines.

But the battlefield has changed substantially since then, notably by the ubiquitous use of Russian surveillance drones and hunter-killer drones. Those weapons have made it more difficult for Ukraine to protect the tanks when they are quickly detected and hunted by Russian drones or rounds.

Five of the 31 tanks have already been lost to Russian attacks.