The leader of jihadist militant group Islamic State in Iraq and the Levant (ISIL or ISIS) has called on Muslims to travel to Iraq and Syria to help build an Islamic state, in an audio message, according to BBC News.

Abu Bakr al-Baghdadi called on Muslims to immigrate to the "Islamic State", saying it was a duty. He made a "special call" for judges, doctors, engineers and people with military and administrative expertise.

ISIL says it is forming an Islamic state, or caliphate, on the territories it controls in Iraq and Syria. In an earlier audio recording this week, the ISIL proclaimed Abu Bakr al-Baghdadi as caliph and "leader for Muslims everywhere". The central government in Baghdad has lost control of vast swathes of territory to Sunni militants, led by ISIL, over the past month. The group says its Islamic state will extend from Aleppo in northern Syria to Diyala province in eastern Iraq. Setting up a state governed under strict Islamic law has long been a goal of many jihadists.

"Rush O Muslims to your state. Yes, it is your state. Rush, because Syria is not for the Syrians, and Iraq is not for the Iraqis," al-Baghdadi said in a new audio message on Tuesday, July 1.

"O Muslims everywhere, whoever is capable of performing hijrah (emigration) to the Islamic State, then let him do so, because hijrah to the land of Islam is obligatory," he added.

He also called on jihadist fighters to escalate fighting during the holy month of Ramadan, which began on Sunday.

"There is no deed in this virtuous month or in any other month better than jihad in the path of Allah, so take advantage of this opportunity and walk the path of you righteous predecessors," he said in the 19-minute audio message.

Little is known about the Isis chief, nicknamed "the invisible sheikh", who unlike al Qaeda leaders such as Osama Bin Laden and Ayman al-Zawahiri, does not appear in video messages.