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.
The vast Anbar province stretches from the western edges of Baghdad all the way to Jordan and Syria to the northwest, and the fighting has greatly disrupted use of the highway linking Baghdad to the Jordanian border, a key artery for goods and passengers.
Al-Maliki's Shiite-dominated government has struggled to push back against the Sunni militants, who have seized large swaths 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, who has led the country since 2006 and has not yet secured a third term after recent parliamentary elections, also has increasingly turned to Iranian-backed Shiite militias and Shiite volunteers to bolster his beleaguered security forces.
The parades in Baghdad and other cities in the mainly Shiite south revealed the depth and diversity of the militias' arsenal, from field artillery and missiles to multiple rocket launchers and heavy machine guns, adding to mounting evidence that Iraq is inching closer to a religious war between Sunnis and Shiites.
Al-Maliki has come under growing pressure to reach out to disaffected Kurds and Sunnis, with many blaming his failure to promote reconciliation for the country's worst crisis since the US military withdrew its forces nearly three years ago.
In Baghdad, about 20,000 militiamen loyal to anti-U.S. Shiite cleric Muqtada al-Sadr, many in military fatigues, marched through the sprawling Shiite Sadr City district, which saw some of the worst fighting between Shiite militias and US soldiers before a cease-fire was reached in 2008 that helped stem the sectarian bloodshed that was pushing the country to the brink of civil war.
Similar parades took place in the southern cities of Amarah and Basra, both strongholds of al-Sadr supporters.
Grand Ayatollah Ali al-Sistani, the most respected voice for Iraq's Shiite majority, who normally stays above the political fray, on Friday joined calls for al-Maliki to reach out to the Kurdish and Sunni minorities.
A day earlier President Barack Obama challenged the prime minister to create a leadership representative of all Iraqis.
Al-Maliki's State of Law bloc won the most seats in the April vote, but his hopes to retain his job have been thrown into doubt, with rivals challenging him from within the broader Shiite alliance.
The US, meanwhile, has been drawn back into the conflict. Obama announced on Thursday he was deploying up to 300 military advisers to help quell the insurgency. They join some 275 troops in and around Iraq to provide security and support for the US Embassy and other American interests.