Sunni fighters led by an al-Qaida breakaway group expanded their offensive in a volatile western province, capturing three strategic towns and the first border crossing with Syria to fall on the Iraqi side.
It is the latest blow against prime minister Nouri Maliki, who is fighting for his political life even as forces beyond his control are pushing the country towards 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 on streets where many of them battled US forces five years 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 group (Isis) 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 towards a key dam in the city of Haditha, which was built in 1986 and has a hydraulic power station that produces some 1,000 megawatts. Destruction of the dam would hamper 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 against a possible attack by the Sunni militants.
Rawah’s mayor, Hussein Ali al-Aujail, said the militants ransacked the town’s government offices and forced local army and police forces to pull out. Rawah and Anah had remained under government control since nearby Fallujah fell to the Sunni militants in January.
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 have already seized the facilities on the Syrian side of the border and several other posts in areas under their control.
Police and the army said the Sunni insurgents seized Qaim and its crossing, about 200 miles west of Baghdad, after killing about 30 Iraqi troops in day-long clashes on Friday. The officials said people were now crossing back and forth freely.
Chief military spokesman Lt Gen Qassim al-Moussawi acknowledged Qaim’s fall, saying troops aided by local tribesmen sought to clear the city of “terrorists”.
Mr Maliki’s Shiite-dominated government has struggled to push back against Islamic extremists and allied Sunni militants who have seized large swathes of the country’s north since taking control of the second-largest city of Mosul on June 10 as Iraqi government forces melted away.
The prime minister has also increasingly turned to Iranian-backed Shiite militias and Shiite volunteers to bolster his beleaguered security forces.