Gunmen took control Tuesday of Al-Qaim, the Iraqi side of a border crossing with Syria, after security forces withdrew, police and army officers said.
The gunmen were loyal to the rebel Free Syrian Army and Al-Qaeda's Syrian franchise Al-Nusra Front, which already controlled the Syrian side of the crossing.
Al-Qaim is one of three official crossings from Iraq into Syria.
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One at Rabia to the north was abandoned by Iraqi forces last week during a major offensive by Sunni militants and taken over by fighters from the Kurdish regional government.
The status of the third one, at Al-Walid, to the southwest of Al-Qaim, was not immediately known.
The offensive, spearheaded by the jihadist Islamic State of Iraq and the Levan, overran all of one province and chunks of three others in a matter of days.
Iraqi security forces performed poorly during the initial offensive, in some cases abandoning uniforms and vehicles to flee.
While they seem to have recovered somewhat from the shock of the onslaught and have retaken certain areas, militants continue to gain ground elsewhere.