KABUL (Reuters) - A deadly mix of hard use and poor maintenance has taken a heavy toll in recent months on Afghanistan’s ageing fleet of Soviet-era helicopters, one of the mainstays of its air support while U.S. Black Hawk helicopters enter service over coming years.

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A crash in southern Afghanistan last month in which two crew members were killed was at least the eighth this year. Earlier incidents have included a crash in the western province of Farah in October in which a deputy army corps commander was killed along with several other senior officials.

As the pace of operations against Taliban and Islamic State insurgents has increased, crews say they have been under pressure to take short cuts with maintenance, an issue U.S. military advisers have highlighted as a major concern for the fledgling Afghan air force they are trying to build up.

Afghan officials say the relentless tempo has pushed pilots and crews to fly overloaded aircraft and carry out non-standard “local maintenance” of problems that can range from repairing bulletholes to fixing engine damage.

“There are 20 operations in the country at one time and we need the choppers to support the ground forces. If they are not flyable, we have to make them flyable,” a senior government official said. “The helicopters are not that old but their maintenance cycle is a big issue.”

Defence Ministry spokesman Ghafoor Ahmad Javed acknowledged there were sometimes technical problems with maintenance, but he said the issue was being addressed.

“It is the problem with maintenance, overuse and difficulties adjusting to weather and we are looking into fixing this,” he said.

The air force currently has 47 Russian-built Mi-17 helicopters, the workhorses of its fleet and all that remains of 62 Mi-17s acquired in 2013. However as of end-September, only 22 were in service, according to a report from U.S. Congressional watchdog SIGAR.

While U.S. UH-60 Black Hawk helicopters will be replacing them over coming years, only 21 of a planned 158 aircraft have so far been delivered while pilots and ground crew are trained to work with the new aircraft.

Afghan forces have been steadily assuming more responsibility for maintenance work and currently manage 90 percent of maintenance on the Mi-17 helicopters. But obtaining parts has become increasingly complicated, a problem that has not been eased by diplomatic tensions between Washington and Moscow.

Afghan pilots also complain that they are forced to rush preparation by commanders with little appreciation of the demands of maintaining the helicopter fleet.

With a small number of aircraft, often-overloaded helicopters fly round-the-clock, delivering ammunition, transporting troops, dropping special forces into combat or evacuating casualties.

“Unfortunately very often our commanders give us orders for tasks without enough time for proper planning of the flight route, location, weather or anything,” said one helicopter pilot, who spoke on condition of anonymity.

“Most of the time, we’re flying with an incomplete plan.”