Sunni fighters have seized a border post on the Iraq-Syria frontier, security sources said, smashing a line drawn by colonial powers a century ago in a campaign to create an Islamic Caliphate from the Mediterranean Sea to Iran.
The militants, led by the Islamic State in Iraq and the Levant (ISIL), first moved into the nearby town of al-Qaim on Friday, pushing out security forces, the sources said on Saturday.
Once border guards heard that al-Qaim had fallen, they left their posts and militants moved in, the sources said.
Sameer al-Shwiali, media adviser to the commander of Iraq's anti-terrorist squad, told Reuters the Iraqi army was still in control of al-Qaim.
Al-Qaim and its neighbouring Syrian counterpart Albukamal are on a strategic supply route. A three-year-old civil war in Syria has left most of eastern Syria in the hands of Sunni militants, now including the Albukamal-Qaim crossing.
The Albukamal gate is run by al Qaeda's official Syria branch, the Nusra Front, which has clashed with ISIL but has sometimes agreed to localised truces when it suits both sides.
The head of the Syrian Observatory for Human Rights monitoring group, Rami Abdulrahman, said ISIL had pushed the Nusra Front out from many areas of eastern Syria in the past few days and their capture of al-Qaim will allow them to quickly move to the Syrian side.
ISIL already controls territory around the Albukamal gate, effectively pinching the Nusra Front between its forces in Syria and those in neighboring Iraq, said Abdulrahman, who tracks the violence.
The al Qaeda offshoot has captured swathes of territory in northwest and central Iraq, including the second city, Mosul. They have seized large amounts of weaponry from the fleeing Iraqi army and looted banks.
World powers are deadlocked over the crises in Iraq and Syria. Shi'ite Iran has said it will not hesitate to protect Shi'ite shrines if asked by Baghdad but Sunni-run Saudi Arabia has warned Tehran to stay out of Iraq.
U.S. President Barack Obama has offered up to 300 U.S. special forces advisers to help the Iraqi government recapture territory seized by ISIL and other Sunni armed groups across northern and western Iraq.
But he has held off granting a request for air strikes to protect the government and renewed a call for Iraq's long-serving Shi'ite prime minister, Nuri al-Maliki, to do more to overcome sectarian divisions that have fuelled resentment among the Sunni minority.
The fighting has divided Iraq along sectarian lines. The Kurds have expanded their zone in the northeast to include the oil city of Kirkuk, which they regard as part of Kurdistan, while Sunnis have taken ground in the west.
The government has mobilized Shi'ite militia to send volunteers to the front lines.