Militants from an al-Qaeda splinter group have launched attacks south of captured Mosul a day after more than 500,000 people fled the city.

Iraqi police said insurgents had seized control of Tikrit and freed hundreds of prisoners, AFP news agency reported.

Some 250 guards at Iraq's largest oil refinery in Baiji were also forced to withdraw to another town after the Islamic State of Iraq and the Levant (ISIL) militants sent a delegation of tribal chiefs to persuade them to pull out.

Video footage also showed fighting on the outskirts of Kirkuk, close to the Kurdistan region.

A police colonel said Tikrit had fallen into the hands of militants on Wednesday. The city lies roughly halfway between Baghdad and second city Mosul, which was captured on Tuesday.

Residents in Baiji said police officers and soldiers were warned not to challenge the fighters, who destroyed a court house and a police station.

Jasim al-Qaisi said "Yesterday at sunset some gunmen contacted the most prominent tribal sheikhs in Baiji and told them 'we are coming to die or control Baiji, so we advise you to ask your sons in the police and army to lay down their weapons and withdraw''".

A senior government official said the production of crude oil in Iraq - which produces around 3.5 million barrels of oil per day - could be affected after the attack on Baiji.

On Tuesday, militants seized control of Mosul, forcing an estimated 500,000 people to flee the area.

Gunmen, wearing military uniforms and all-black clothing, have been guarding banks and government buildings in the city and calling over loudspeakers for government employees to return to work.

The militants also seized the Turkish consulate in the city and reportedly kidnapped the head of the diplomatic mission and 48 staff members, according to police.

Families have made their way into the Kurdistan region, which has its own military force the Peshmerga.

One police officer said "We can't beat them. We can't. They are well trained in street fighting and we're not. We need a whole army to drive them out of Mosul. They're like ghosts: they appear, strike and disappear in seconds".

Ninevah province governor Atheel al-Nujaifi said on Wednesday authorities had a plan to restore security.

UN urges unity

Iraqi Prime Minister Nouri al-Maliki has asked parliament to declare a state of emergency that would grant him additional powers to tackle the crisis. He also appealed for help from the international community.

And Iraq's Foreign Minister, Hoshyar Zebari, has said Baghdad will co-operate with Kurdish forces to "flush out these foreign fighters".

Influential Shiite cleric Moqtada al-Sadr has called for the formation of militia units to help defend religious sites in Iraq.

ISIL has made serious gains in Iraq in the last year, seizing control of the city of Falluja and parts of Ramadi. The group is also fighting President Bashar al-Assad's regime in Syria.

Led by Abu Bakr al Baghdadi, it was formed after a split with al Qaeda's international leader Ayman al Zawahiri.

Earlier, the United Nations Secretary-General urged all political leaders in Iraq to show national unity. Ban Ki-moon expressed grave concern about the "serious deterioration" of security in Mosul, and condemned the recent spate of terrorist attacks that have left scores of dead and wounded in Iraq’s northern and eastern
provinces.

In a statement issued by his spokesperson in New York, the UN chief said he is gravely concerned by the deteriorating situation in Mosul.

The Secretary-General "strongly condemned" the terrorist attacks in restive Anbar, Baghdad, Diyala, Ninewa, and Salah al-Din provinces that have killed and wounded scores of civilians over the past several days.

“The Secretary-General urges all political leaders to show national unity against the threats facing Iraq, which can only be addressed on the basis of the Constitution and within the democratic political process" said the statement.

Mr. Ban also encouraged the government of Iraq and the Kurdish regional government to cooperate in restoring security to Ninewa Province and in delivering urgently needed humanitarian aid.

"The United Nations Assistance Mission for Iraq (UNAMI) stands ready to support these efforts" the statement adds.