BAGHDAD -- A Shiite militia seized control of an Iraqi town for the first time Saturday, blunting the advance toward Baghdad of radical Sunni fighters in a sign that the widespread mobilization of paramilitary forces may be starting to have an impact.
The first overtly sectarian battle of the new war unfolding for control of Iraq came as thousands of heavily armed Shiites paraded defiantly through the streets of Baghdad, responding to a call to arms from their most revered cleric.
Iraqi soldiers and police sealed off streets and stood guard as the procession of thousands of armed men passed, a humiliating reminder of the extent to which the U.S.-trained security forces have lost stature since the mass desertions across the north that facilitated the advance of the al-Qaida-inspired Islamic State of Iraq and Syria.
Thousands of Shiites flocked for a third straight day to volunteer to fight, crowding recruitment centers set up in mosques, a former airport and at the homes of prominent Shiite clerics and politicians. Their numbers swelled in response to the call to jihad issued Friday by Grand Ayatollah Ali Sistani, regarded as the most influential Shiite cleric in the world.
As President Barack Obama considered military options to meet Iraq's request for assistance, Defense Secretary Chuck Hagel on Saturday ordered a U.S. aircraft carrier into the Persian Gulf.
The USS George H.W. Bush was expected to arrive in the gulf from the Arabian Sea by Saturday night, along with the guided-missile cruiser USS Philippine Sea and guided-missile destroyer USS Truxtun, Rear Adm. John Kirby, the Pentagon press secretary, said in a statement.
An Iraqi general told reporters in Baghdad that the armed forces have "regained the initiative" in recent days and are now confident that Baghdad is secure.
Maj. Gen. Qassim al-Moussawi said the military is coordinating with forces in the city of Samarra and other areas north of the capital to retake territory claimed by the insurgents "and have achieved remarkable victories with the help of volunteers."
In the town of Muqdadiyah in Diyala province northeast of Baghdad, residents said the Asaib Ahl al-Haq militia had made the difference and that fighters with that Iranian-backed paramilitary movement were in control.
The Iraqi security forces retreated when ISIS fighters swept into the town, repeating a pattern witnessed in numerous Iraqi towns overrun in the past week. The militia then pushed into the town from the south, forced the ISIS fighters back and took control, said a resident who spoke on the condition of anonymity out of fear for his life.
The Sunni and Shiite groups are now facing each other on the northern outskirts of the town, he said, marking a new front line in what is fast becoming a Sunni-Shiite war.
Both sides in the conflict claimed Saturday to have inflicted large numbers of casualties. ISIS said on its official Twitter account that it had executed 1,700 Shiite soldiers Friday. Meanwhile, the pro-government Iraqia satellite channel reported that Iraqi forces had killed nearly 300 "terrorists" across northern Iraq on Saturday. It was impossible to verify either claim.
Speaking in Tehran, Iranian President Hassan Rouhani would not confirm or deny reports that the Quds Force, an elite unit of Iran's Revolutionary Guard Corps, had entered Iraq to assist Prime Minister Nouri al-Maliki's government. He told reporters that his country is prepared to step in to help Iraq but that Iraqi authorities had not requested assistance.
"We are ready to help Iraq within the framework of international law, and if the Iraqi government and nation ask us to do so, we will consider it," he said.
Separately, however, Iran's interior minister, Abdolreza Rahmani Fazli, said the country has "no intention of sending military forces to Iraq."
"We'll provide consultation and guidance to Iraqi forces," Mr. Fazli told reporters in Tehran, according to the semiofficial Mehr News Agency.
Iraqi security forces claimed Saturday to have regained control of the small town of Ishaqi north of Baghdad, and claims were reported in local news media that the Iraqi air force had struck a number of targets in insurgent-occupied areas.