The latest assaults saw the security forces making "tactical" withdrawals in the face of an insurgent onslaught that has displaced hundreds of thousands and alarmed the world amid fears Iraq could tear itself apart.
The militants, led by the jihadist Islamic State of Iraq and the Levant (ISIL), seized the towns of Rawa and Ana after taking the Al-Qaim border crossing yesterday, residents said.
The government said its forces had made a "tactical" withdrawal from the towns, control of which has allowed the militants to open up a strategic route to neighbouring Syria, where they also control swathes of countryside along the Euphrates river valley.
Washington wants Arab states to bring pressure on Iraq's leaders to speed up government formation, which has made little headway since elections in April.
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While American leaders have stopped short of calling for Prime Minister Nuri al-Maliki to step down -- arguing that it is up to the Iraqis to choose their own leaders -- they have left little doubt that they feel the Shiite premier has squandered the opportunity to rebuild his country since US troops withdrew in 2011.
"Unfortunately what we've seen is a breakdown of trust."
The seizure of Al-Qaim leaves just one of three official border crossings with Syria in the hands of the federal government. The third is controlled by Kurdish forces.
Anti-government fighters already hold areas of the western desert province of Anbar, which abuts the Syrian border, after taking all of one city and parts of another earlier this year.
Elsewhere, Iraqi government forces, fighting back against the insurgents after initially wilting before their onslaught, today launched an air strike on the militant-held city of Tikrit, killing at least seven people, residents of the city said.