By SAMEER N. YACOUB and ADAM SCHRECK
BAGHDAD (AP) -- A representative for Iraq's top Shiite cleric on Friday urged Iraqis to defend their country as militants who have seized large swaths of the nation's Sunni heartland captured two towns in an ethnically mixed province northeast of Baghdad.
Neighboring Shiite powerhouse Iran signaled its willingness to confront the growing threat from this week's militant blitz, which the United Nations estimates has claimed hundreds of lives.
The fresh gains by insurgents, spearheaded by fighters from the al-Qaida-inspired Islamic State of Iraq and the Levant, come as Prime Minister Nouri al-Maliki's Shiite-led government struggles 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.
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.
Rupert Colville, the spokesman for the U.N. human rights office, told journalists in Geneva that the number of people killed in recent days may run into the hundreds, and that the number of wounded could approach 1,000.
He said top U.N. rights official Navi Pillay plans to issue a statement later today expressing alarm at the rapid deterioration of security in Iraq, and said the office has received reports that Iraqi army soldiers as well as 17 civilians in a single street in Mosul had been rounded up and killed by militants.
The 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.
Iran's official IRNA news agency reported Friday that former members of Tehran's powerful Revolutionary Guard have announced their readiness to fight in Iraq against the Islamic State, while Iranian state television quoted President Hassan Rouhani as saying his country will do all it can to fight 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.
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.
Shiite cleric Sheik Abdul-Mahdi al-Karbalaie told worshippers Friday it is a civic duty to confront the threat facing Iraq. He represents Grand Ayatollah Ali al-Sistani, the most revered Shiite spiritual leader in Iraq.
"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," al-Karbalaie said.
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 out. After seizing the town, the gunmen announced through 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 with its large Shiite population, the capital would be a far more difficult target.
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 in Baghdad on Thursday after authorities urged Iraqis to help battle the insurgents.
Trumpeting their victory, the Islamic militants also declared they would impose Shariah law in Mosul, which they captured on Tuesday, and other areas they seized.
A video posted online showed Islamic State 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.
President Barack Obama said Iraq will need more help from the United States, but he did not specify what it would be willing to provide. Senior U.S. officials who spoke on condition of anonymity because they were not authorized to discuss the matter by name said Washington is considering whether to conduct drone missions in Iraq.
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 in 2011 - 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.