Afghanistan’s military is fielding UH-60 Black Hawk helicopters that lack the lift capacity and flight capabilities of the Russian Mi-17s it currently flies, which are also simpler to fly and maintain than the U.S.-made aircraft, according to a U.S. Defense Department watchdog report.

The Defense Department is replacing the Mi-17 with Black Hawks to provide additional lift and aerial fires capability to the Afghan Air Force (AAF). Congress also called for the switch from buying Russian helicopters to purchasing aircraft from U.S. manufacturers.

That transition will present challenges for the AAF as it continues to battle Taliban insurgents and the illicit narcotics economy, according to the most recent quarterly report on the war by DoD’s Lead Inspector General for Operation Freedom’s Sentinel.

Black Hawks, which are made by Lockheed Martin Sikorsky [LMT], do not have the lift capacity of Mi-17s and cannot fly at altitudes encountered in Afghanistan’s remote mountainous regions from which insurgents operate, according to the report, published in late May. The report covers the period January through March 31, 2018.

“The transition presents several challenges that have yet to be fully addressed,” the report says. “They are unable to accommodate some of the larger cargo items the Mi-17s can carry, and in general, it takes almost two Black Hawks to carry the load of a single Mi-17. Furthermore, unlike Mi-17s, Black Hawks cannot fly at high elevations and, as such, cannot operate in remote regions of Afghanistan where Mi-17s operate.”

According to 9th Air and Space Expeditionary Task Force-Afghanistan, the Mi-17s will play a “crucial role” in the near term fighting season, but as they phase out of service, the challenges presented by transitioning to the UH-60 will become more pronounced.

By the end of 2019, the Mi-17 inventory is scheduled to be reduced from 47 (24 of which are in long-term maintenance) to 20. The fleet size is scheduled to decrease to 18 by the end of 2021 and then down to 12 by the end of 2022.

Between December 2017 and March 2018, the U.S. issued four UH-60s to the AAF at a cost of $13.6 million, according to the Special Inspector General For Afghanistan Reconstruction (SIGAR). The Defense Department set aside $709.8 million in fiscal 2018, the second year of the transition effort.

As of March 2018, eight Black Hawks had been delivered to the AAF, with another 45 purchased but not yet fielded, out of a total 159 planned as part of the Afghan National Defense and Security Force’s AAF modernization road map. The AAF in 2018 has trained 20 UH-60 Pilots and 16 special mission operators.

As of March 3, the U.S. spent $6.4 billion to support and develop the AAF from fiscal years 2010 to 2018, with roughly $1.4 billion appropriated in fiscal 2018, according to SIGAR.

The AAF’s current inventory of aircraft includes the eight UH-60 utility helicopters; four Mi-35 helicopters, of which four are unavailable; 47 Mi-17s, of which 24 are unavailable; 25 MD530 helicopters, of which five are unavailable; 24 C-208 utility airplanes, four C-130 transport airplanes, 20 A-29 light attack airplanes, SIGAR counted in its most recent quarterly report to Congress.