BAGHDAD (AFP) - The leader of a brutal jihadist group called on Muslims with military, medical and managerial skills to immigrate to its newly-declared pan-Islamic state, in an audio recording released on Tuesday.

"Those who can immigrate to the Islamic State should immigrate, as immigration to the house of Islam is a duty," said Abu Bakr al-Baghdadi.

Baghdadi heads the Islamic State (IS) jihadist group which on the weekend declared a "caliphate" in territories it controls in Iraq and Syria.

The newly named "caliph" said the appeal especially applied to "judges and those who have military and managerial and service skills, and doctors and engineers in all fields."

Baghdadi also addressed the group's fighters, saying that "your brothers in all the world are waiting" to be rescued by them.

The call comes after IS on Sunday declared a "caliphate," an Islamic form of government last seen under the Ottoman Empire, and ordered Muslims worldwide to pledge allegiance to Baghdadi.

The militant group, which operates in both Iraq and Syria, said their caliphate would spread from Aleppo in northern Syria to Diyala in eastern Iraq, and ordered Muslims in those areas to "obey" and pledge allegiance to their new leader.