KABUL (Reuters) - The Afghan Air Force took delivery of its first four U.S.-made UH-60 Black Hawk helicopters on Tuesday as part of a planned replacement of its aging fleet of Russian-made Mi-17 helicopters, officials said. The U.S. plans to supply a total of 159 Black Hawks to Afghanistan in the coming years as it boosts the capacity of the AAF, considered one of the best-performing parts of the Afghan Defence and Security Forces.  The first Afghan Air Force UH-60s arrived today in Kandahar. They will help further develop a capable and sustainable AAF,  the NATO-led Resolute Support mission said. The robust Soviet-era Mi-17 is the workhorse of the AAF, accounting for around half of all sorties in recent months and is very popular with Afghan pilots.  As well as moving troops and casualties and flying in supplies to areas difficult to reach by road, the helicopters can also be fitted with weapons for close air support to units on the ground. However their age makes them increasingly difficult to maintain and replace and they are to be phased out over the coming years and replaced with refurbished Black Hawks, one of the mainstays of the U.S. Army s helicopter force. According to a report in July by the Special Inspector General for Afghanistan, a U.S. Congressional watchdog, 19 of the AAF s 46-strong Mi-17 fleet were unusable because they were undergoing overhaul or repair. In addition to operational considerations, the Pentagon has also long faced political pressure from Congress to switch spending to U.S.-built aircraft. Strengthening the Afghan Air Force is a central part of President Ashraf Ghani s four year plan to improve Afghan security forces which have struggled to contain the Taliban insurgency since the NATO-led coalition backing the government ended its main combat operations in 2014. 