BAGHDAD (AFP) - Iraqi Prime Minister Nuri al-Maliki travelled to the embattled city of Samarra on Friday for a security meeting, Governor Ahmed Abdullah said.

Militants attacked the city, located 110km north of Baghdad, earlier in the week, and witnesses said they were readying for another assault on Friday.

The city houses the revered Shi'ite Al-Askari shrine, which was bombed by militants in 2006, sparking a bloody Sunni-Shi'ite sectarian war that killed tens of thousands.

A major militant offensive, spearheaded by powerful jihadist group the Islamic State of Iraq and the Levant (ISIL), has overrun all of one province and chunks of three more since Monday.

Security forces have so far failed to halt the push, with some abandoning their vehicles and positions and discarding their uniforms.

Meanwhile, leading Shi'ite cleric Grand Ayatollah Ali al-Sistani urged Iraqis on Friday to take up arms against Sunni militants marching on Baghdad, as thousands volunteered to bolster the capital's defences.

Ayatollah Sistani's call to defend the country against the offensive spearheaded by the ISIL came as United States President Barack Obama said he was exploring all options to save Iraq's security forces from collapse.

The United Nations reported a spate of summary executions by ISIL fighters in its campaign, which captured Iraq's second city Mosul on Tuesday, before heading south towards Baghdad.

"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," Sistani's representative announced during the main weekly prayers in the Shi'ite shrine city of Karbala.

"He who sacrifices for the cause of defending his country and his family and his honour will be a martyr."

The elderly Ayatollah Sistani, who rarely appears in public, is highly influential in the Shi'ite Muslim world and is adored by millions.

Mr Obama said Iraq would need "more help from the United States and from the international community" to bolster security forces that Washington spent billions of dollars training and equipping before withdrawing its own troops in 2011.

"Our national security team is looking at all the options... I don't rule out anything," he said.

One option is drone strikes, like those controversially deployed in Afghanistan, Pakistan and Yemen, a US official told AFP.

But there is currently no plan to send ground troops back to Iraq, where around 4,500 American soldiers died between the US-led invasion in 2003 and the withdrawal of US forces in 2011.

US Secretary of State John Kerry urged Iraqi politicians to "come together and to show unity", and said Washington was aiding the beleaguered government with air surveillance.

Iranian President Hassan Rouhani pledged his government's full support against "terrorism".

Despite their many differences, Teheran and Washington are united in their determination to prevent Iraq following its western neighbour Syria into civil war.

In an interview, Iraqi Foreign Minister Hoshyar Zebari likened the performance of security forces in the face of the offensive to the collapse of Sunni Arab dictator Saddam Hussein's army in 2003.

They "took off military uniforms and put on civilian clothes and went to their houses, leaving weapons and equipment" behind, he said.

The Interior Ministry has adopted a new security plan for Baghdad, its spokesman Brigadier General Saad Maan told AFP.

"We have been in a war with terrorism for a while, and today the situation is exceptional," he said.

Technicians said the communications ministry had blocked access to social media websites including YouTube, Facebook and micro-blogging site Twitter.

Militants and their supporters have been active on sites including Twitter during the offensive.

North of Baghdad, militants were gathering for a new attempt to take Samarra, home to a revered Shiite shrine whose 2006 bombing sparked a deadly sectarian war.

Residents of Samarra, just 110km from Baghdad, said gunmen were gathering to the north, east and south-east of the city.

Militants already mounted two assaults on Samarra, one on Wednesday and one late last week, which were thwarted after heavy fighting.

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 late Monday, have pressed south into ethnically divided Diyala province.

On Friday, they were battling pro-government forces near Muqdadiyah, just 80km from Baghdad's city limits.

With militants closing in on the capital, forces from the autonomous Kurdish region took control of a territory they have sought to rule for decades against the objections of successive governments in Baghdad.

Kurdish security forces moved into the strategic Saadiyah and Jalawla districts of Diyala province overnight after the army withdrew, Deputy Governor Furat al-Tamimi said.

Diyala's mixed Arab, Kurdish, Sunni and Shiite population has made the province a byword for violence since Saddam's 2003 overthrow.

Kurdish forces took control of the ethnically divided northern oil city of Kirkuk on Thursday.

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.

The swift collapse of Baghdad's control comes on top of the loss of Fallujah, west of the capital, earlier this year. It has been a blow for Western governments that paid a steep price both in lives and money in Iraq.

UN human rights chief Navi Pillary condemned reports of summary executions by ISIL and the displacement of as many as half a million people in northern Iraq.

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 Friday that 40,000 people have fled Tikrit and Samarra, adding to half a million people believed to have fled Mosul.