The militants took the crossing yesterday, a colonel and a captain in the border guards said.
Security forces that had been guarding it headed south to join troops at another crossing with Jordan, they said.
The two other official crossings between Iraq and Syria, Al-Qaim and Rabia, are also outside government hands, with militants holding the first and security forces from the country's autonomous Kurdish region in control of the second.
A major militant offensive, led by jihadists from the Islamic State of Iraq and the Levant but involving other groups as well, has overrun swathes of five Iraqi provinces since it was launched on June 9.
Security forces wilted under the initial onslaught, and are now struggling to hold their ground in the face of the relentless militant drive.
