WASHINGTON, July 29 (Reuters) - Taliban assassinations of Afghan pilots detailed by Reuters this month mark another "worrisome development" for the Afghan Air Force as it reels from a surge in fighting, a U.S. government watchdog said in a report released on Thursday. At least seven Afghan pilots have been assassinated off base in recent months, two senior Afghan government officials told Reuters, part of what the Islamist Taliban says is a campaign to see U.S.-trained Afghan pilots "targeted and eliminated." As the United States prepares to formally end its 20-year military mission in Afghanistan on Aug. 31, Taliban insurgents are quickly seizing territory once controlled by the U.S.-backed government of President Ashraf Ghani, raising fears they could eventually try to take the capital Kabul. The Special Inspector General for Afghan Reconstruction (SIGAR), in its quarterly report to Congress covering the three-month period through June, broadly portrayed an Afghan Air Force (AAF) under growing strain from battling the Taliban amid the U.S. withdrawal - and becoming less ready to fight. The AAF"s fleet of UH-60 Black Hawk helicopters, for example, had a 39 percent readiness rate in June, about half the level of April and May. All Afghan airframes were flying at least 25 percent over their recommended scheduled-maintenance intervals, SIGAR reported. "All aircraft platforms are overtaxed due to increased requests for close air support, intelligence, surveillance, reconnaissance missions and aerial resupply now that the (Afghan military) largely lacks U.S. air support," it wrote. At the same time, air crews remain over-tasked due to the deteriorating security situation in Afghanistan and a pace of operations that "has only increased”, it said. SIGAR also cited the Reuters report. "Another worrisome development concerning AAF aircrew was a (Reuters) report that the Taliban is deliberately targeting Afghan pilots," it said, before presenting the Reuters" findings without additional comment. Along with Afghanistan"s Special Forces, the Afghan Air Force is a pillar of the nation"s strategy for preventing a Taliban takeover of cities. But special operations forces are also being misused, SIGAR reported. It said most Afghan National Army corps refuse to execute missions without support from its elite commandos. Citing NATO data, SIGAR said that when Afghan commandos arrive, they are misused to perform tasks intended for conventional forces, including route clearance and checkpoint security. Still, the report cautioned that it was difficult to evaluate what constituted military misuse of the elite forces when the Afghan government "is fighting for its existence".