The Ukrainian military has opted to withdraw its US-supplied M1 Abrams main battle tanks from the front lines due to the threat posed by Russian drones, American officials told the Associated Press. Kiev has reportedly lost several of the tanks since their long-delayed deployment in February.

The decision came as Moscow increasingly relies on UAVs for round-the-clock surveillance and air strikes, making the Abrams “too difficult… to operate without detection or coming under attack,” the AP reported on Thursday. The tanks have been removed from the fight for an unspecified period as Kiev and Washington work to “reset tactics,” Joint Chiefs of Staff Vice Chairman Adm. Christopher Grady told the outlet.

When you think about the way the fight has evolved, massed armor in an environment where unmanned aerial systems are ubiquitous can be at risk,” Grady added. “Now, there is a way to do it. We’ll work with our Ukrainian partners, and other partners on the ground, to help them think through how they might use that, in that kind of changed environment now, where everything is seen immediately.”

Though the United States agreed to provide 31 Abrams in January 2023, the armor took months to arrive, and even longer before they first saw combat in February. Ukraine has so far lost at least five of the tanks, according to the AP, with Russian forces claiming several kills since the Abrams was first spotted on the battlefield. At least one of the tanks was reportedly crippled with a cheaply made kamikaze drone, part of Moscow’s growing fleet of surveillance and armed UAVs.

The ubiquity of drones in the conflict has meant “there isn’t open ground that you can just drive across without fear of detection,” another senior defense official told the AP, explaining the decision to sideline the Abrams.

The move comes as Washington steps up assistance to Ukrainian troops following a lull in support, with Congress passing a massive $61 billion military aid package last weekend after months of gridlock in the House.

The Pentagon immediately approved a $1 billion shipment on Wednesday, including artillery rounds, air defense missiles, infantry fighting vehicles and a long list of other gear – the largest round of US aid in several months. Another $6 billion weapons package was announced on Friday which is to include munitions for the US-made Patriot air defense system.

Following a failed counteroffensive last summer, Kiev has repeatedly urged for additional weapons from its NATO sponsors, warning it would continue to lose ground to Russian forces without Western-made aircraft, armor and long-range missiles.

On Wednesday, the US State Department revealed it had already sent Ukraine its first shipment of longer-range ATACMS missiles last month, saying the decision was not announced “in order to maintain operational security for Ukraine at their request.”