Last Updated May 14, 2015 4:41 PM EDT
BEIRUT -- The Islamic State of Iraq and Syria (ISIS) 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 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.
CBS News' Khaled Wassef reports that in previous speeches, al-Baghdadi always called on Muslims to flock to the group's self-styled caliphate and take up arms against the apostates and the Shiites. Here, for the first time, he seems to be giving his followers the choice to either join ISIS in Syria and Iraq, or just carry out attacks at home.
"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.
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. In that message, he urged his followers to "explode the volcanoes of jihad everywhere."
The reclusive Al-Baghdadi has made only one public appearance since declaring himself caliph, delivering a July sermon at a mosque in Mosul.
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.
In February, CBS News obtained a photo of al-Baghdadi while he served time at a U.S. military prison in Iraq in 2004. A previous CBS News investigation revealed that Baghdadi was among at least 12 of the top leaders of ISIS who served time at Bucca, one of the toughest, American prisons in Iraq. Baghdadi spent 10 months at the prison, from February to December 2004.