BAGHDAD -- 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 UN 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 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 neighbouring 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, travelled to the city of Samarra to meet with military commanders late Friday, state TV reported.
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 95 kilometres 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 it was their civic duty to confront the threat.
"Citizens who can carry weapons and fight the terrorists in defence 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 Iraq faces "great danger," and fighting the militants "is everybody's responsibility and is not limited to one specific sect or group."
In Geneva, UN 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 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 crossfire, 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.
Neighbouring Shiite powerhouse Iran signalled 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 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 kilometres northeast of Baghdad, and Sadiyah, 95 kilometres 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.
-- The Associated Press