Militants seized the Iraqi city of Tikrit on Wednesday but security forces thwarted an assault on Samarra as a lightning jihadist offensive launched in second city Mosul swept closer to Baghdad.

Since the Islamic State of Iraq and the Levant began its spectacular assault in Mosul late on Monday, terrorists 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 militants have advanced since their seizure on Tuesday of Mosul, a city of two million people, has sent off alarms in Western capitals.

It has also triggered a hostage crisis for Turkey, which threatened harsh reprisals if 49 Turks seized by the jihadists at its consulate in the main northern city were harmed in any way.

Tehran and Washington, which despite their many differences have a shared interest in preventing Iraq following neighboring Syria into all-out civil war, both pledged more aid to Baghdad.

The jihadists vowed on Twitter that they will "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 half way between Baghdad and Mosul.

Another officer 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 70 miles from the capital.

State television said security forces responded with air strikes, and residents said the fighting subsided without the militants entering the city.

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

In his weekly address Wednesday, Prime Minister Nouri al-Maliki merely renewed his call to arm civilians to resist the jihadists.

Maliki urged Nineveh's residents "and its tribes to stand with the army and police."

Washington has warned that the jihadists threaten the entire region and promised more aid for the Baghdad government.

State Department spokeswoman Jen Psaki said the United States is committed to "working with the Iraqi government and leaders across Iraq to support a unified approach against 9the jihadists') continued aggression."

In Tehran, Foreign Minister Mohammad Javad Zarif said Shiite Iran "offers its support to the government and people of Iraq against terrorism."

The swift collapse of Baghdad's control, which comes on top of the loss of Fallujah, west of Baghdad, at the start of the year, has been a blow for Western governments that invested lives and money in the invasion that toppled Saddam in 2003.

However, Foreign Secretary William Hague said there is "no question" of British troops being sent back to Iraq.

The International Organisation for Migration said 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 49 people including the head of the mission and three children, a Turkish official said.

They were in addition to 31 Turkish truck drivers seized at a Mosul power station.

The hostage-taking drew a stark warning from Turkish Foreign Minister Ahmet Davutoglu.

"All those involved should know that if our citizens are harmed in any way, they will be the subject of harsh reprisals," he said.

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