TEHRAN, Iran (AP) — Iran and the United States share a common enemy in the Islamic State militant group, but a deep-seated lack of trust has so far kept the longtime foes from publicly joining hands in a coalition to defeat the extremists.

Their inability to work together complicates efforts to beat back the extremists that both Washington and Tehran see as a threat, and has left Iraq’s new government — which considers both countries allies — scratching its head as it tries to tackle the most serious threat to its stability since American troops left in 2011.