WASHINGTON (AFP) - Iran is stepping up its support for Shiite militants in Iraq, supplying them with more sophisticated weapons, the top US military officer said Thursday. Admiral Mike Mullen said Iran had made a decision to curtail its support for Shiite factions in 2008 but has now increased its activity in Iraq, sending in lethal arms that were being used against American forces. "Iran is very directly supporting extremist troops which are killing our troops," Mullen told reporters at a Pentagon Press Association luncheon.