A small number of Ukrainians have been trained in the US on how to operate killer “Switchblade” drones, single-use weapons that fly into their targets and detonate on impact, a senior US defense official disclosed on Wednesday.

The Ukrainians undergoing training on the Switchblades and other weaponry number less than a dozen. They had arrived in the US for regular military education programs prior to Russia’s invasion of Ukraine on Feb. 24.

“We took advantage of the opportunity to pull them aside for a couple of days and provide them some training, particularly on the switchblades UAV,” the senior US defense official told reporters, speaking on condition of anonymity. “UAV” refers to an unmanned aerial vehicle.

The US withdrew its military advisers from Ukraine ahead of Russia’s invasion, seeking to avoid a direct military confrontation between US and Russian forces that could escalate into a broader war.

As a result of the withdrawal, the US and NATO have largely constrained their provision of weaponry to Ukraine to systems that Ukrainian forces knew how to operate prior to Russia’s invasion.

That includes US weapons that have given Ukraine an edge against Russian forces, like Javelin anti-tank missiles and portable Stinger surface-to-air missiles that can target Russian aircraft. It also includes Soviet-era systems that are still in the inventories of some NATO nations.

But Switchblades, which are relatively easy-to-use and could be highly effective in attacking Russian ground forces, had not been part of training packages prior to Russia’s invasion. The drones are made by AeroVironment Inc.

In recent testimony, the assistant secretary of defense for international security affairs, Celeste Wallander, said the US had committed to sending Ukraine 100 Switchblade systems.

Defense Secretary Lloyd Austin said on Tuesday that the Pentagon is sending Ukraine two variants of the Switchblade, including one with an anti-armor warhead.

“The Switchblade 600 and 300 will move as quickly as they possibly can,” Austin told the House of Representatives Armed Services Committee.

Ukrainians are expected to quickly use the first 100 systems sent.

“I’m convinced that when we get the first set of Switchblades in, there will be an immediate request from the Ukrainians for more,” the top US commander in Europe, Air Force General Tod Wolters, told Congress on March 30.

The senior US official declined to say on Wednesday where in the US the training of Ukrainians was taking place or offer more information on other weapons systems they’re being trained on.

“Our expectation is that these individuals will be heading back into Ukraine relatively soon as they were originally anyway,” the official told reporters.