A US military helicopter has crashed in Afghanistan, killing all five American crew members.The Chinook chopper came down in the central district of the country, where it was transporting troops in support of an anti-militant operation. The army said it was too early to say if hostile fire or mechanical failure was to blame. It was not the first crash of a US Chinook in Afghanistan. In June, another Chinook was shot down during an anti-guerrilla mission in the eastern province of Kunar, killing all 16 troops on board. In April, a CH-47 crashed during a dust storm in Ghanzi province, killing 15 American servicemen and three US civilian contractors.