Islamist militants are closing in on Baghdad after capturing two towns north of Iraq's capital.
Officials said Iraqi soldiers abandoned their posts with no resistance when fighters from the Islamic State of Iraq and the Levant (Isis) pushed into Diyala province.
Militants driving machine gun-mounted pickups entered the towns of Jalula and Sadiyah on Thursday night, police said. Jalula is 80 miles from Baghdad and Sadiyah is 60 miles away.
Sunni fundamentalist fighters have vowed to capture Baghdad and Shia holy cities further south after overrunning Mosul, Iraq’s second-largest city, and driving the army out of northern provinces.
The UN said hundreds have been killed - with militants carrying out summary executions of civilians in Mosul, including 17 civilians in one street.
A dozen Iraqi security personnel were also killed and four women committed suicide after being raped.
UNAMI, the UN's mission to Iraq, estimates that almost 1,000 people may have been injured in recent days and half a million residents have fled Mosul.
A court employee in the Dawasa area of central Mosul was executed, said spokesman Rupert Colville.
Prisoners released by the militants from Mosul prison had been looking to exact revenge on those responsible for their incarceration and some went to Tikrit and killed seven former prison officers there, Mr Colville said.
“We've also had reports suggesting that government forces have also committed excesses, in particular the shelling of civilian areas on 6 and 8 June in Mosul, resulting in a large number of civilian casualties,” he added.
“There are claims that up to 30 civilians may have been killed during this shelling.”
The UN has also been told that Iraqi government forces were not allowing people to leave Mosul at one point and Isis are now using their own checkpoints to target dissenters.
Iraq's Prime Minister, Nouri al-Maliki, has so far been unable to form a coherent response the al-Qaida-inspired group as the power of his Shia-led government evaporates in parts of the country.
He failed an attempt for parliament to declare a state of emergency on Thursday and has asked Barack Obama for help to combat the growing insurgency.
The US President said he was "ruling nothing out" but David Cameron and William Hague have stressed that the UK will not be sending in troops.
It is the biggest threat to Iraq’s stability since the withdrawal of US forces in 2011 and has pushed it nearer the prospect of partition into Sunni, Shia and Kurdish zones.
The rise of Isis, which wants to impose Sharia law, raises the prospect of more sectarian violence and ethnic cleansing. Much of Baghdad was cleansed of its Sunni population in 2005 and 2006.
A representative for Iraq's top Shia cleric has called on Iraqis to defend their country by joining security forces to battle the militants.
Sheik Abdul-Mahdi al-Karbalaie said it is “a duty” for citizens to defend against “the dangers threatening Iraq”.
Abu Mohamed al-Adnani, a spokesman for Isis, said that the Shia, 60 per cent of the Iraqi population, “are a disgraced people”, accusing them of being “polytheists”.
Security officials said the fighters managed to take control of two weapons depots holding 400,000 weapons, including AK-47 rifles, rockets and rocket-propelled grenades, artillery shells and mortars.
A quarter of the stockpiles were sent to Syria, they said.
Saddam Hussein's hometown of Tikrit was overrun by the militants on Wednesday and witnesses said fighters raised posters of the late dictator and Izzat Ibrahim al-Douri, his former deputy who escaped the 2003 invasion and has eluded security forces ever since.
The nine-year Shia dominance over Iraq, established after the US, Britain and other allies overthrew Saddam Hussein, may be coming to an end.
The power vacuum in the northern city of Kirkuk has been filled by Kurdish security forces, who have taken over an airbase and other abandoned military posts.
Americans were evacuated from an Iraqi air base north of Baghdad amid the panic and Germany has urged citizens to leave parts of the country, including Baghdad, immediately.
President Obama said Iraq will need more help from the United States, but he did not specify what it would be willing to provide. Officials said Washington is considering drone missions.
“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,” he said in Washington.
The UN Security Council met on the crisis, underscoring the growing international alarm over the advances by Isis.
William Hague, the Foreign Secretary, said Britain could offer assistance with counter-terrorism expertise to authorities in Iraq.
Speaking after talks on Iraq with US Secretary of State John Kerry in London, Mr Hague stressed that Britain has no intention of sending troops into the country.
But he said that a team from the Department for International Development was now on the ground in northern Iraq to see what humanitarian help the UK can give, and made clear that Britain is also ready to advise the Baghdad administration.
Mr Hague also urged Iraq's political leaders from different communities to unite in responding to the “brutal aggression against their country”, and said that Britain would continue to press at the United Nations for a concerted international response.
Additional reporting by AP and Reuters
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