The Islamic State of Iraq and al-Sham (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.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.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.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.