US President Barack Obama was considering options short of sending troops to help Iraq to counter a Sunni militant onslaught as a leading cleric called Shiites to take up arms to defend the country.
The militants captured Iraq's second city of Mosul on Tuesday in a lightning offensive, before advancing south towards Baghdad, leading Obama to say he was "looking at all the options".
But he told reporters at the White House on Friday that putting troops on the ground was not on the table.
"We will not be sending US troops back into combat in Iraq, but I have asked my national security team to prepare a range of other options that could help support Iraqi security forces," he said.
His comments came after Shiite Grand Ayatollah Ali al-Sistani urged Iraqis to defend the country against the offensive spearheaded by the jihadist Islamic State of Iraq and the Levant (ISIL).
A representative of Sistani, who is adored by Shiites but rarely appears in public, made the call from the Shiite shrine city of Karbala.
"Citizens who are able to bear arms and fight terrorists, defending their country and their people and their holy places, should volunteer and join the security forces to achieve this holy purpose," the representative said.
Prime Minister Nuri al-Maliki, who travelled to the embattled city of Samarra north of Baghdad, said in a statement that security forces "began their work to clear all our dear cities from these terrorists".
Washington spent billions of dollars training and equipping Iraqi security forces before withdrawing its troops in 2011.
Around 4,500 American soldiers died between the US-led invasion in 2003 and the withdrawal.
Obama said that while the United States was willing to help out, Iraq needed to take steps to heal its sectarian divide.
"The United States will not involve itself in military action in the absence of a political plan by the Iraqis that gives us some assurance that they're prepared to work together," Obama said.
"We won't 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, as soon as we're not there, suddenly people end up acting in ways that are not conducive to the long-term stability of the country."
"Any action that we may take to provide assistance to Iraqi security forces has to be joined by a serious and sincere effort by Iraq's leaders to set aside sectarian differences."
Pentagon spokesman Rear Admiral John Kirby declined to say what kind of response was being prepared.
He confirmed that the US aircraft carrier USS George H.W. Bush and its strike group were in the region and ready to act. The US navy said the carrier group was in the Arabian Sea.
Kirby also said the US military had stepped up intelligence sharing with Iraq "in recent days at the request of the Iraqi government."
US officials blamed Iraqi leaders for failing to strengthen and support the nation's military after US troops withdrew despite billions of American dollars poured into training and equipping the army.
State Department deputy spokeswoman Marie Harf dismissed criticism from Republican lawmakers that a residual US force would have stopped the Iraqi army from collapsing.
"When we left Iraq, after years of sacrifice and American taxpayer money, and certainly our troops felt that sacrifice more than anyone, the Iraqis had an opportunity," Harf told reporters.
"We had helped their security forces. We had helped their army," she insisted.
"We had gotten them on their feet and helped build their capacity, and quite frankly, they did not take advantage of that opportunity."
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Instead, Iraqi leaders "created a climate where there were vulnerabilities when it came to the cohesion of the Iraqi army," Harf said.
- Iran pledges support -
President Hassan Rouhani of Shiite Iran, which borders Iraq, pledged his government's full support against "terrorism".
Despite their differences, Tehran and Washington are united in their determination to prevent Iraq following its western neighbour Syria into civil war.
But Washington said it had not begun talks on Iraq with Tehran.
In an interview Friday, Iraqi Foreign Minister Hoshyar Zebari likened the performance of security forces during the militant offensive to the collapse of Sunni Arab leader Saddam Hussein's army in 2003.
However, the interior ministry said it had adopted a new security plan for Baghdad.
Technicians said the communications ministry had blocked access to social media websites, among them YouTube, Facebook and micro-blogging site Twitter.
Militants were gathering for a new attempt to take Samarra, just 110 kilometres (70 miles) north of Baghdad and home to a revered Shiite shrine whose 2006 bombing sparked a deadly sectarian war.
The militants, who have taken a huge swathe of predominantly Sunni Arab territory in northern and north-central Iraq since launching their offensive in Mosul Monday, have pressed south into ethnically divided Diyala province.
On Friday, they battled pro-government forces near Muqdadiyah, just 80 kilometres (50 miles) from Baghdad's city limits.
- Kurdish forces move in -
Forces from the autonomous Kurdish region, meanwhile, took control of territory they have sought to rule for decades against the objections of successive governments in Baghdad.
It has been the fulfilment of a decades-old Kurdish ambition to expand their autonomous region in the north to incorporate historically Kurdish-majority territory across northern and north-central Iraq.
UN human rights chief Navi Pillay condemned reports of summary executions by ISIL.
The UN said it had received reports of women committing suicide after being raped or forced to marry ISIL fighters and the summary execution of people believed to have worked for the police.
The International Organisation for Migration estimated that 40,000 people have fled Tikrit and Samarra, adding to half a million people believed to have fled Mosul.