Islamic State of Iraq and Levant (Isis) insurgents are transporting US-supplied Humvees across the border into war-torn Syria after capturing Iraq's second city of Mosul.

Militants were seen riding in the vehicles around Mosul and other areas under their control but photos have emerged on social media outlets of the military vehicles being inspected by senior Isis commander Umar al Shishani in Syria.

Shishani, who was born in Georgia and acts as commander of the northen sector of Isis in Syria, is considered one of the most influential leaders of the Syrian opposition after a series of assaults on military bases near Aleppo.

The US State Department has released a statement saying that it is "deeply concerned" by the Islamist militants' siege of Mosul.

"The situation remains extremely serious. Senior U.S. officials in both Washington and Baghdad are tracking events closely in coordination with the Government of Iraq," the statement read.

"The United States stands with the Iraqi people," it continued.

The capture of Mosul occurred after an overnight assault by hundreds of Islamic State in Iraq and the Levant (Isis) jihadists, bearing rocket-propelled grenades, sniper rifles and machine guns, on key local government, police and military headquarters.

The militants freed up to 1,000 inmates from Mosul's central prison, according to senior police officials. The group are now in control of Mosul airport and local television stations.

Iraqi Prime Minister Nouri al-Maliki has called on Iraq's parliament to adopt martial law across Iraq after a televised speech where he said "Iraq is undergoing a difficult stage" and that insurgents had captured "vital areas in Mosul".

He urged the public and government to unite "to confront this vicious attack, which will spare no Iraqi."