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 troubles.
Fighters from the al-Qaida-inspired Islamic State of Iraq and the Levant made fresh gains, capturing two towns in an ethnically mixed province northeast of Baghdad. The ISIL assault also 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-era figures and other disaffected Sunnis, has emerged as the biggest threat to Iraq's stability since the U.S. withdrawal at the end of 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's Shiite-led government is struggling to form a coherent response after militants overran the country's second-largest city of Mosul, Saddam Hussein's hometown of Tikrit and smaller communities, as well as military and police bases — often after meeting little resistance from state security forces.
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, a cleric whose comments are thought to reflect al-Sistani's thinking.
He warned that Iraq faced ''great danger'' and that the responsibility of fighting the militants ''is everybody's responsibility, and is not limited to one specific sect or group.''
In Geneva, U.N. human rights chief Navi Pillay warned of ''murder of all kinds'' and other war crimes in Iraq, where her office says the number of those killed in recent days may run into the hundreds and the number of wounded could approach 1,000.
Pillay said her office has received reports that militants rounded up and killed Iraqi army soldiers as well as 17 civilians in a single street in Mosul.
Her office is hearing of ''summary executions and extrajudicial killings'' as ISIL militants overran Iraqi cities and towns this week, the statement said.
''I am extremely concerned about the acute vulnerability of civilians caught in the cross-fire, or targeted in direct attacks by armed groups, or trapped in areas under the control of ISIL and their allies,'' Pillay said in a statement. ''And I am especially concerned about the risk to vulnerable groups, minorities, women and children.''
Obama did not specify what options he was considering, but he ruled out sending American troops back into combat in Iraq.
''We're not going to allow ourselves to be dragged back into a situation in which, while we're there we're keeping a lid on things, and after enormous sacrifices by us, after we're not there, people start acting in ways that are not conducive to the long-term stability and prosperity of the country,'' Obama said from the South Lawn of the White House.
Neighboring Shiite powerhouse Iran signaled its willingness to confront the growing threat from the militant blitz.
Iran's official IRNA news agency reported that former members of Tehran's powerful Revolutionary Guard have announced their readiness to fight in Iraq against the Islamic State, and Iranian state TV quoted President Hassan Rouhani as saying his country will do all it can to battle terrorism next door.
''The Islamic Republic of Iran will apply all its efforts on the international and regional levels to confront terrorism,'' the report said Rouhani told al-Maliki by phone.
Iranian officials denied reports their forces were actively operating inside Iraq, however.
Iranian lawmaker Mansour Haghighatpour, who sits on an influential parliamentary committee on national security and foreign policy, told The Associated Press that Baghdad is capable of fighting the militants, but that Tehran would consider other options if asked.
Iran has built close political and economic ties with postwar Iraq, and many influential Iraqi Shiites have lived for stretches of time in the Islamic Republic. Iran earlier this week halted flights to Baghdad because of security concerns and said it was intensifying security measures along its borders.
Police officials said militants driving in machine gun-mounted pickups entered the two newly conquered towns in Diyala province late Thursday — Jalula, 125 kilometers (80 miles) northeast of Baghdad, and Sadiyah, 95 kilometers (60 miles) north of the Iraqi capital. Iraqi soldiers abandoned their posts there without any resistance, they said.
Residents of Jalula said the gunmen issued an ultimatum to the Iraqi soldiers not to resist and give up their weapons in exchange for safe passage. After seizing the town, the gunmen announced on loudspeakers that they have come to rescue residents from injustice and that none would be hurt.
The officials spoke on condition of anonymity because they were not authorized to talk to journalists, and the residents declined to give their names because of fears for their safety.
The Islamic State has vowed to march on Baghdad, but the capital would be a far more difficult target with its large Shiite population.
So far, the militants have stuck to the Sunni heartland and former Sunni insurgent strongholds where people are already alienated by al-Maliki's government over allegations of discrimination and mistreatment. The militants also would likely meet far stronger resistance, not only from government forces but by Shiite militias.
Shiite cleric Muqtada al-Sadr and the Asaib Ahl al-Haq Shiite militia vowed to defend Shiite holy sites, raising the specter of street clashes and sectarian killings.
Still, Baghdad authorities have tightened security around the capital and residents are stocking up on essentials. Hundreds of young men crowded in front of the main army recruiting center Thursday after authorities urged Iraqis to help battle the insurgents.
The Islamic militants also declared they would impose Shariah law in Mosul, which they captured on Tuesday, and other areas they seized.
Online video showed ISIL fighters holding a parade in a Mosul neighborhood, with many of the gunmen cruising in armored vehicles seized from Iraqi forces.
A fighter using a loudspeaker urged the people to join the militant group ''to liberate Baghdad and Jerusalem.'' The Islamic State's black banners adorned many of the captured vehicles. Some in the crowd shouted ''God is with you'' to the fighters.
The video appeared authentic and consistent with Associated Press reporting of the events depicted.
In northern Iraq, Kurdish security forces have moved to fill the power vacuum caused by the retreating Iraqi forces — taking over an air base and other posts abandoned by the military in the ethnically mixed city of Kirkuk.
Three planeloads of Americans were being evacuated from a major Iraqi air base in Sunni territory north of Baghdad, U.S. officials said Thursday, and Germany urged its citizens to immediately leave parts of Iraq, including Baghdad.
The advances by the Sunni militants are a heavy defeat for al-Maliki. His Shiite-dominated political bloc came first in April parliamentary elections — the first since the U.S. military withdrawal — but failed to gain a majority, forcing him to try to build a governing coalition.
''We do have a stake in making sure that these jihadists are not getting a permanent foothold in either Iraq or Syria, for that matter,'' Obama said in Washington.
Al-Maliki and other Iraqi leaders have pleaded with the Obama administration for more than a year for additional help to combat the growing insurgency.
Schreck reported from Dubai, United Arab Emirates. Associated Press writers Julie Pace in Washington, John Heilprin in Geneva and Nasser Karimi in Tehran, Iran, contributed to this report.