The advance in the last two days dealt another blow to Prime Minister Nouri al-Maliki, who is fighting for his political life even as forces beyond his control are pushing the country toward a sectarian showdown.
In a reflection of the bitter divide, thousands of heavily armed Shiite militiamen, eager to take on the Sunni insurgents, marched through Iraqi cities in military-style parades yesterday on streets where many of them battled US forces a half decade ago.
The towns of Qaim, Rawah and Anah are the first territory seized in predominantly Sunni Anbar province, west of Baghdad, since fighters from the Islamic State of Iraq and the Levant overran the city of Fallujah and parts of the provincial capital of Ramadi earlier this year.
The capture of Rawah on the Euphrates River and the nearby town of Anah appeared to be part of march toward a key dam in the city of Haditha, the destruction of which would damage the country's electrical grid and cause major flooding.
Iraqi military officials said more than 2,000 troops were quickly dispatched to the site of the dam to protect it. They spoke on condition of anonymity because they were not authorised to speak to the media.
The Islamic State's Sunni militants have carved out a large fiefdom along the Iraqi-Syrian border and have long travelled back and forth with ease, but control over crossings like that one in Qaim allows them to more easily move weapons and heavy equipment to different battlefields.
Syrian rebels already have seized the facilities on the Syrian side of the border and several other posts in areas under their control.