Half a million people flee as ISIS militants seize the Iraqi cities of Tikrit and Mosul

Militants seized the Iraqi city of Tikrit Wednesday but their assault on Samarra was repulsed as a lightning jihadist offensive launched in second city Mosul swept closer to Baghdad.

Since the Islamic State of Iraq and the Levant began their spectacular assault in Mosul late Monday, militants have captured a large swathe of northern and north-central Iraq, prompting as many as half a million people to flee their homes.

The speed with which ISIL and its allies have advanced after their seizure Tuesday of Mosul -- a city of two million people -- has sent alarm bells ringing not only in Baghdad but in Western capitals.

It has also triggered a hostage crisis for Ankara, which is scrambling to secure the release of 48 Turks taken hostage by the jihadists.

In a statement on Twitter, ISIL vowed that it would "not stop this series of blessed invasions" that has seen the fall of the whole of Nineveh province in the north and swathes of Kirkuk and Saleheddin provinces further south.

Tikrit -- hometown of executed dictator Saddam Hussein -- was the second provincial capital to fall in as many days as the jihadists and their allies captured a string of mainly Sunni Arab towns where resentment against the Shiite-led government runs deep.

"All of Tikrit is in the hands of the militants," a police colonel said of the Salaheddin provincial capital, which lies roughly half way between Baghdad and Mosul.

A police major said the militants had freed some 300 inmates from a prison there.

After Tikrit's fall, the operation spread down the main highway towards Baghdad, with militants battling security forces on the northern outskirts of Samarra, just 110 kilometres (70 miles) from the capital.

Iraqiya state television said security forces launched air strikes on them, and witnesses said the clashes ended without the militants entering the city.

It was not immediately clear what became of the attackers.

Militants had already tried to seize the city late last week, and were only halted by a massive deployment of troops, backed by tribal militia and air power.

Although Samarra too is mainly Sunni Arab, it is super sensitive for the government as it is home to a shrine revered by the country's Shiite majority.

A 2006 bombing of the mausoleum by Al-Qaeda sparked a Shiite-Sunni sectarian conflict that left tens of thousands dead.

In other developments, a series of bombings, including a suicide attack on tribal leaders in Baghdad, killed 37 people in Shiite areas of central and southern Iraq, officials said.

The lightning advance poses significant challenges to Baghdad, with the New York-based Eurasia Group risk consultancy saying they would be bolstered by cash from Mosul's banks, hardware from military bases and hundreds of men they freed from prison.

Prime Minister Nuri al-Maliki has responded by asking parliament to declare emergency rule and saying citizens would be armed to fight them.

Washington has warned that ISIL threatened the entire region.

The nominee as the next US ambassador to Iraq, Stuart Jones, told a lawmakers it was "one of the most dangerous terrorist groups in the world."

He also said Washington would "work with our international partners to try to meet the needs of those who have been displaced."

- Exodus of civilians -

The International Organisation for Migration said its sources in Mosul estimated the violence leading up to the jihadists' takeover "displaced over 500,000 people in and around the city."

On Wednesday, gunmen in military uniforms and all-black clothing guarded government buildings and banks in the city, residents told AFP by telephone.

Militants stormed the Turkish consulate and kidnapped 48 people including the head of the mission, a Turkish official said.

"Forty-eight Turks including the consul, staff members, guards and three children were abducted," the official told AFP, speaking on condition of anonymity.

"All are doing well."

Militants called over loudspeakers for government employees to go back to work.

"I did not open the door of the shop since last Thursday because of the security conditions," said Abu Ahmed, a 30-year-old shopkeeper.

'Afraid about freedoms'

Bassam Mohammed, a 25-year-old student, said he would not join the exodus of residents leaving the city.

"But I am afraid about freedoms, and I am especially afraid that they will impose new laws on us," he said.

Known for its ruthless tactics and suicide bombers, ISIL is arguably the most capable force fighting President Bashar al-Assad inside Syria as well as the most powerful militant group in Iraq.

ISIL is led by the shadowy Abu Bakr al-Baghdadi and backed by thousands of Islamist fighters in Syria and Iraq, many of them Westerners.

It appears to be surpassing Al-Qaeda as the world's most dangerous jihadist group.

On Wednesday, the Syrian government said it was ready to help Baghdad in its fight against "terrorism," while the rebel Free Syrian Army called for support from Arab states for its own battle against ISIL in Syria.

Iraqi officials said the offensive would have no impact on the government's vital oil earnings, as sabotage to the main pipeline has meant there have been no exports from northern oil fields around the city of Kirkuk for weeks.

"We haven't any production from the region -- all our production is from the south," said Oil Minister Abdul Karim Luaiby.

Iraqi exports averaged around 2.58 million barrels per day in May, despite the closure of the pipeline from the Kirkuk oilfields to the Turkish Mediterranean port of Ceyhan, which can carry 500,000 bpd.