BAGHDAD — The Islamic State on Saturday released an audio recording of a speech purportedly made by the organization’s leader, Abu Bakr al-Baghdadi, in which he implored more Muslims to fight on behalf of his increasingly besieged group.

The authenticity of the recording could not be confirmed. It appeared to be Mr. Baghdadi’s first public address since May, and it followed reports that he had been either killed or gravely wounded by airstrikes.

The speech was released on jihadist social media accounts after a string of defeats for the Islamic State in Iraq and amid intensifying bombardment by an American-led military coalition in both Syria and Iraq.

In the speech, Mr. Baghdadi acknowledged escalating pressure on the Islamic State, also referred to as ISIS or ISIL, but framed it as worldwide attack by “disbelievers” against Muslims.