(CBS News) BAGHDAD - Sunni militants have seized another town in Iraq's western Anbar province, the fourth to fall in two days, officials said Sunday, in what is shaping up to be a major offensive in one of Iraq's most restive regions.
The officials said the militants captured Rutba, about 90 miles east of the Jordanian border, late Saturday. Residents were on Sunday negotiating with the militants to leave after an army unit on the town's outskirts threatened to start shelling.
The latest advance has dealt another blow to Shiite 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 Saturday on streets where many of them battled U.S. forces a half decade ago.
The towns of Qaim, Rawah, Anah and Rutba are the first seized in predominantly Sunni Anbar province since fighters from the Islamic State of Iraq and the Levant overran the city of Fallujahand 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.
Taking Rutba gives the insurgents control over the final stretch of a major highway to neighboring Jordan, a key artery for passengers and goods that has been infrequently used for months because of deteriorating security.
Rutba has a population of 40,000 but it has recently been home to 20,000 displaced from Fallujah and Ramadi.
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 authorized to speak to the media.
The Islamic State and allied militants have carved out a large fiefdom along the Iraqi-Syrian border. Control over crossings like that one in Qaim allows them to more easily move weapons and heavy equipment. Rebels control the Syrian side of the crossing.
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 troops melted away.
The prime minister, who has led the country since 2006 and has not yet secured a third term after recent parliamentary elections, has increasingly turned to Iranian-backed Shiite militias and 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 Sunni and Shiite Muslims.
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 U.S. 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 U.S. 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 U.S., meanwhile, has been drawn back into the conflict. Obama announced 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 U.S. Embassy and other American interests.
Obama has been adamant that U.S. troops would not be returning to combat, but has said he could approve "targeted and precise" strikes requested by Baghdad.
Iraq enjoyed several years of relative calm before violence spiked a year ago after al-Maliki moved to crush a Sunni protest movement against alleged discrimination and abuse at the hands of his government and security forces.
On Saturday four separate explosions killed 10 people, including two policemen, and wounded 22 in Baghdad, according to police and hospital officials. And in an incident harkening back to the peak of sectarian killings in 2006 and 2007, two bodies, presumably of Sunnis, were found riddled with bullets in Baghdad's Shiite district of Zafaraniyah, police and morgue officials said.
All officials spoke on condition of anonymity because they were not authorized to speak to journalists.