The crash of the F-15E came in a month that has proved the deadliest for Western troops in the course of the nearly eight-year-old Afghanistan conflict. At least 50 coalition service members, including 26 Americans, have been killed so far in July. The highest previous monthly tally for Western troop fatalities in Afghanistan was 46. American military officials have said the ongoing buildup of U.S. troops, together with a push into insurgent-controlled areas, probably will exact a rising casualty toll in coming months. The forces of Britain and Canada, two crucial allies in the NATO coalition, also have seen a spike in fatalities this month, which has set off wrenching domestic debate in both countries about where the effectiveness of Western war strategy in Afghanistan. The cause of the U.S. jet's crash was being investigated, but military authorities said they had ruled out the possibility that it was shot down. 'The crash was not due to hostile fire,' U.S. military officials said in a statement. Afghan officials in Ghazni province said U.S. troops had sealed off the scene of the crash. The fighter jet, known as a Strike Eagle, is a modified version of the supersonic F-15E. It is often used to provide 'close air support' -- that is, to hit ground targets during combat operations. But U.S. military officials said they were not aware of any clashes in Ghazni at the time of the crash. The military did not disclose what the fighter jet's mission was, other than that it was 'conducting coalition operations.' Insurgents in Afghanistan on rare occasions have been able to bring down helicopters, but there is no known instance of them shooting down a fighter plane. Even losses of such sophisticated craft because of weather or mechanical failure are rare. A crash last week of a helicopter carrying private military contractors in Helmand province, in Afghanistan's south, is under investigation. All six of those aboard that chopper, identified as Ukrainian nationals, were killed. The Moldovan contractor said the helicopter was shot down, and the Taliban claimed responsibility. Helmand has been the scene of a wide-ranging offensive this month spearheaded by U.S. and British forces. Eastern Afghanistan, where the fighter jet crashed, also has been a trouble spot for much of the summer. It borders Pakistan's volatile tribal areas, where Pakistan's military has been carrying out operations against the Taliban in recent weeks. 