Sunni Arab militants have seized the Al-Waleed border post between Iraq and Syria, officers said on Monday, meaning that all crossings between the neighbouring countries are now outside government control.
The militants took the crossing on Sunday, 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.
Iraqi Prime Minister Nuri al-Maliki's security spokesman, Lieutenant General Qassem Atta, was later quoted by state television as saying that government forces had retaken Al-Waleed.
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But two officials who work at the crossing said it was still held by militants.
The two other official border posts between Iraq and Syria -- Al-Qaim and Rabia -- are also outside government hands, Militants control the first while security forces from the country's autonomous Kurdish region hold the second.
With the seizure of Al-Waleed, the militants have increased their ability to bring men and material across the border from Syria and have tightened their control over western Iraq.
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.