TEHRAN—Iran's elite military unit, the Islamic Revolutionary Guard Corps, has transferred lethal new munitions to its allies in Iraq and Afghanistan in recent months, according to senior U.S. officials, in a bid to accelerate the U.S. withdrawals from these countries.

The Revolutionary Guard has smuggled rocket-assisted exploding projectiles to its militia allies in Iraq, weapons that have already resulted in the deaths of American troops, defense officials said. They said Iranians have also given long-range rockets to the...