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."
In Geneva, U.N. human rights chief Navi Pillay warned of "murder of all kinds" and other war crimes in Iraq, and said the number killed in recent days may run into the hundreds, while the 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 heard 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. "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 on the South Lawn of the White House.
Administration officials said Obama is weighing airstrikes using drones or manned aircraft. Other short-term options include an increase in surveillance and intelligence-gathering. The U.S. also is likely to increase aid to Iraq, including funding, training and both lethal and non-lethal equipment.
Al-Maliki and other Iraqi leaders have pleaded with Washington for more than a year for additional help to combat the growing insurgency.
Neighboring Shiite powerhouse Iran signaled its willingness to confront the growing threat from the militant blitz.
Former members of Tehran's powerful Revolutionary Guard have announced their readiness to fight in Iraq against the Islamic State, the official IRNA news agency reported. 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 their forces were actively operating in Iraq, however.
Mansour Haghighatpour, who sits on an influential Iranian parliamentary committee on national security and foreign policy, told The Associated Press that Baghdad is capable of fighting the militants, but Tehran would consider other options if asked.
Iran has built close political and economic ties with postwar Iraq, and many influential Iraqi Shiites have spent time in the Islamic Republic. Iran this week halted flights to Baghdad because of security concerns and said it was intensifying security on its borders.
Police said Sunni militants driving machine gun-mounted pickups entered the two newly conquered Iraqi towns in Diyala province late Thursday — Jalula, 125 kilometers (80 miles) northeast of Baghdad, and Sadiyah, 95 kilometers (60 miles) north of the capital. Iraqi soldiers abandoned their posts there without any resistance, they said.
Jalula residents said the gunmen issued an ultimatum to the 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 gunmen later disappeared from Jalula, only to be replaced with the Kurdish security forces known as peshmerga. They raised the Kurdish flag over government buildings and transferred abandoned Iraqi military equipment back to the Kurds' self-ruled northern region, according to two police officials. The officials spoke on condition of anonymity because they were not authorized to talk to journalists, and the residents declined to give their names out 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. The militants would face far stronger resistance from government forces and Shiite militias.
So far, they have stuck to the Sunni heartland and former Sunni insurgent strongholds where people are alienated by al-Maliki's government over allegations of discrimination and mistreatment.
Iraq's former Sunni vice president, Tariq al-Hashemi, told the AP in Istanbul that while the Islamic State was one player in the uprising, they are not the driving force.
"They are not involved in the decision-making," he said, adding that the Sunni tribes in Mosul and Anbar are "behind this Iraqi spring."
Baghdad considers al-Hashemi a fugitive after he was found guilty in absentia in terrorism-related cases — charges he dismisses as politically motivated.
Shiite cleric Muqtada al-Sadr and the Asaib Ahl al-Haq Shiite militia have vowed to defend Shiite holy sites, raising the specter of street clashes and sectarian killings.
Still, authorities have tightened security around the capital and residents stocked up on supplies.
Hundreds of young men volunteered for military service at a recruiting center Thursday, and more were being urged to join by cars playing Shiite religious songs that roamed Shiite neighborhoods Friday after the cleric's call.
The Islamic militants in Mosul declared they would impose Shariah law and trumpeted their success in a parade of seized armored vehicles that was captured on online video.
A fighter with a loudspeaker urged the people to join the militants "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 AP reporting of the events depicted.
The U.N. refugee agency reported that local authorities say 300,000 people fleeing from Mosul have sought safety in the Erbil and Duhok governorates in the Kurdistan region. UNHCR monitoring teams report many arrived with little more than what they were wearing, although some are staying with relatives and in hotels, the agency said.
Kurdish security forces filled the power vacuum caused by the retreating Iraqi forces, taking control of the ethnically mixed oil hub of Kirkuk in northern 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 — but failed to get a majority, forcing him to try to build a governing coalition.
The U.N. envoy in Iraq, Nickolay Mladenov, urged the Federal Court to certify the election results before the current parliament's mandate expires Saturday.
"There is a need to guarantee the continuity of the parliament, representing all Iraqis, is in place and will continue to address urgent decisions of national importance," Mladenov said.
Iraq's government began blocking access to websites like Facebook and Twitter, according to Renesys, a New Hampshire-based Internet analysis firm. The outages, reported Thursday and Friday, appeared to coincide with government efforts to disrupt the militants' offensive and mirrored other past efforts by Middle East countries to block Internet access.
Internet access routed through Kurdistan into neighboring Turkey appeared to continue functioning, said Jim Cowie, the head of research and development at Renesys. Iraq also accesses the Internet through providers in Jordan and via submarine cables.
"There can always be battle damage but in this particular case it's government directed," Cowie told the AP.