DUBAI (REUTERS) - Five Islamist fighters identifying themselves as Australian and British nationals have called on Muslims to join the war in Syria in a new video released by the Islamic State in Iraq and the Levant (ISIL).

The online footage, titled "There is No Life Without Jihad", showed the five militants sitting in a rural area and holding rifles with Al-Qaeda's black flag flying behind them.

"We are a State who is implementing the syaria (Islamic law) in both Iraq and Sham (Syria), and look at the soldiers - we understand no borders," one fighter said in English.

A caption said he was Abu Muthanna al-Yemeni from Britain. "We have participated in battles in Sham, and we will go to Iraq in a few days, and we will fight there, Allah permitting, and come back, and we will even go to Jordan and Lebanon, with no problems."

The other militants were identified as Abu Bara' al-Hindi and Abu Dujana al-Hindi, also from Britain, and Abu Yahya al-Shami and Abu Nour al-Iraqi, both from Australia.

With stunning speed, ISIL, an offshoot of Al-Qaeda, has captured swathes of territory in north-west and central Iraq, including the second city of Mosul, seizing large amounts of weaponry from the fleeing Iraqi army and looting banks.

President Barack Obama has offered up to 300 Americans to help coordinate the Baghdad government's battle against the Sunni insurgent group.

ISIL, which is also fighting in neighbouring Syria, includes thousands of foreign fighters and has become a magnet for jihadi volunteers from Europe and North Africa, Western intelligence agencies say.

Earlier this month, nine European countries endorsed plans to step up intelligence-sharing and take down radical websites to try to stop European citizens going to fight in Syria and bringing violence back home with them.

British Prime Minister David Cameron said on Wednesday that Sunni Islamist insurgents fighting in Iraq were planning to attack Britain and that "ungoverned spaces" where militants thrived had to be shut down.