The audio address, titled "Even if the Disbelievers Despise Such," after an Islamic verse, purports to have been given by Abu Bakr Baghdadi, the self-styled "caliph" of the militant organization. It was issued by the group's media arm.
The recording quickly galvanized Islamic State supporters and watchers who inundated jihadi online forums and social media with links to the speech.
In the address, the speaker vows that the militants "will never leave fighting, even if only one soldier remains."
Perhaps in a nod to its international "membership," as well as to its media savvy, the group issued transcripts in English and Russian, as well as the original Arabic.
The speech is the first purported message from Baghdadi, known as Caliph Ibrahim to his followers, to emerge since Iraqi government officials reported that he may have been killed or seriously injured in one or more U.S. airstrikes late last week. U.S. Central Command acknowledged conducting strikes against a "gathering" of Islamic State leaders, but did not confirm that Baghdadi was believed to be among those present.
It was not immediately clear whether the address was recorded before or after last week's airstrikes.
In the 16-minute speech, the man identified as Baghdadi boasts of the growing number of jihadist factions who have "pledged allegiance" to the Islamic State, most recently in Libya and Yemen but also in Egypt and Algeria.
He encourages his fighters to "erupt volcanoes of jihad everywhere" and to "light the Earth with fire upon all the tyrants, their soldiers, and supporters." He exhorts Muslims worldwide to fight under the group's banner, adding that it was not only obligatory but also "the best of deeds and the peak of Islam."
The speaker also taunts the members of the U.S.-led coalition, calling them "terrified, weak and powerless" and saying plans to prevent the group's advance by using proxy forces had failed.
"Soon, the Jews and Crusaders will be forced to come down to the ground and send their ground forces to their deaths and destruction," he rails. He adds that "in actuality this has already begun," in reference to the presence in Iraq of some 1,400 U.S. service personnel in what the Pentagon calls advisory and other non-combat roles.
The address is the first attributed to Baghdadi since his much-publicized purported appearance in July at a mosque in Mosul, the Islamic State's stronghold in northern Iraq.
The group, originally formed in Syria, seized Mosul along with large swaths of northern and western Iraq in a lightning advance in June that stunned Iraqi government forces. The group's territorial gains spurred an intense aerial bombing campaign by the United States and a number of Western and regional allies.
Special correspondent Bulos reported from Amman and Times staff writer McDonnell from Beirut.