KABUL - A joint US-Afghan force called in an airstrike on what turned out to be an Afghan army post after taking fire from there before dawn Saturday, killing four Afghan soldiers and prompting an angry demand for punishment from the country's defense ministry. Both NATO and Afghan authorities described the clash around a snow-covered outpost in Wardak province southwest of Kabul as a case of mistaken identity. NATO called the attack "unfortunate" and promised a full investigation.Nevertheless, the deadly strike threatens to strain relations between NATO and the Afghan government at a time when both sides are calling for closer partnership in the fight against the Taliban. The fighting came on the heels of several cases of bloodshed between Afghans and Americans in recent weeks. NATO and Afghan officials said an Afghan interpreter angry over "job issues" shot and killed two U.S. soldiers Friday before he was gunned down by an American service member in the same district as the airstrike. NATO officials said the two attacks appeared unrelated. Saturday's fighting erupted about 3 a.m. when a group of US Special Forces and Afghan commandos approached a remote Afghan army outpost that was set up about 18 months ago to guard the main highway between Kabul and Kandahar. NATO said the Afghan soldiers believed the unit was the Taliban and started shooting. The joint force returned fire and called in the airstrike, which killed the four Afghan soldiers, NATO and the Afghan Defense Ministry said. Seven Afghan soldiers at the checkpoint were wounded, provincial spokesman Shahidullah Shahid said.