BAGHDAD (AP) -- Iraq's Shiite clerical leadership Friday called on all Iraqis to defend their country from Sunni militants who have seized large swaths of territory, and a U.N. official expressed "extreme alarm" at reprisal killings in the offensive, citing reports of hundreds of dead and wounded.
U.S. President Barack Obama said he is weighing options for countering the insurgency, but warned Iraqi leaders that he would not take military action unless they moved to address the country's political divisions.
Fighters from the al-Qaida-inspired Islamic State of Iraq and the Levant made fresh gains, driving government forces at least temporarily from two towns in an ethnically mixed province northeast of Baghdad. The assault threatens to embroil Iraq more deeply in a wider regional conflict feeding off the chaos caused by the civil war in neighboring Syria.
The fast-moving rebellion, which also draws support from former Saddam Hussein-era figures and other disaffected Sunnis, has emerged as the biggest threat to Iraq's stability since the U.S. withdrawal in 2011. It has pushed the nation closer to a precipice that could partition it into Sunni, Shiite and Kurdish zones.
Prime Minister Nouri al-Maliki, whose Shiite-led government is struggling to form a coherent response to the crisis, traveled to the city of Samarra to meet with military commanders late Friday, according to state TV.
Militants earlier in the week overran military bases and several communities including the second-largest city of Mosul and Saddam's hometown of Tikrit. Samarra, the site of a prominent Shiite shrine 60 miles (95 kilometers) north of Baghdad, sits between Tikrit and the capital.
A representative for Grand Ayatollah Ali al-Sistani, the most revered Shiite spiritual leader in Iraq, told worshippers at Friday prayers that it was their civic duty to confront the threat.
"Citizens who can carry weapons and fight the terrorists in defense of their country, its people and its holy sites should volunteer and join the security forces," said Sheik Abdul-Mahdi al-Karbalaie, whose comments are thought to reflect al-Sistani's thinking.
He warned that Iraq faced "great danger," and that fighting the militants "is everybody's responsibility, and is not limited to one specific sect or group."