A helicopter operated by the NATO-led International Security Assistance Force crashed in southern Afghanistan, the alliance said. All four on board were believed dead, according to a US military spokesman who was quoted as saying by CNN. The four US personnel on board were presumed killed, the spokesman said. Enemy action had not been ruled out as a cause for the crash, he was reported as saying. The Black Hawk aircraft came down in bad weather Thursday while attempting to pick up a group of soldiers, The ISAF confirmed the incident and said the crash site had been secured. It did not release any further information about the crash or the fate of those on board.