The Islamic State group on Thursday released an audio message purportedly from its leader, Abu Bakr al-Baghdadi, who has not been seen or heard from in months.

The 35-minute audio message posted on militant websites features a voice that sounds like al-Baghdadi's exhorting all Muslims to take up arms and fight on behalf of the group's self-styled caliphate.

The speaker references the Saudi-led air campaign against Shiite rebels in Yemen, which began on March 26, and harshly criticizes the Saudi royal family.

"Islam was never a religion of peace. Islam is the religion of fighting," he said.

"No one should believe that the war that we are waging is the war of the Islamic State. It is the war of all Muslims, but the Islamic State is spearheading it. It is the war of Muslims against infidels."

"O Muslims go to war everywhere. It is the duty of every Muslim," the speaker said

It was not immediately possible to verify whether the voice was al-Baghdadi's.

In another indication the message was recorded recently, the speaker appears to refer to the thousands of people who fled Ramadi last month as the Islamic State group advanced on the town in Iraq's western Anbar province.

"If some of your relatives are fighting against the religion of God and are loyal to Rawafid and Crusaders we will not hurt you," he said using a derogatory term to refer to Shiites. He called on Sunni members of the Iraqi police and army to repent because "when the hands of the mujahideen get you no repentance will be accepted."

He praised fighters who joined the group in Egypt, Libya, Yemen, Algeria and Tunisia. He welcomed pledges of allegiance from Islamic State supporters in Yemen, Afghanistan and West Africa.

The last audio message purportedly from al-Baghdadi came in November, days after Iraqi officials said he was wounded in an airstrike on an Iraqi town near the Syrian border. Media reports have also said he was severely wounded. The audio message was not accompanied by photos or video.

Al-Baghdadi has only appeared in public once, in a video showing him delivering a Friday sermon in the northern Iraqi city of Mosul last July, shortly after it was captured by his group.

The IS group controls much of northern Iraq and northern and eastern Syria, and has established a self-styled caliphate governed by a harsh and violent interpretation of Islamic law.

The 35-minute audio message, which quoted several verses from the Quran, was released with written English, Russian, Turkish, French and German translations.

In the latest message, al-Baghdadi blasted Arab rulers, calling them "guarding dogs" and saying the Yemen war will lead to the end of the Saudi royal family's rule.