BAGHDAD (AFP) - The Islamic State (IS) group on Thursday released an audio recording it said was of chief Abu Bakr al-Baghdadi, days after air strikes on militant leaders in Iraq sparked rumours he was wounded or killed.

In the 17-minute message, the man purported to be Baghdadi made no direct reference to the strike against IS leadership but mentioned developments that have occurred since.

"Be assured, O Muslims, for your state is good and in the best condition," he said, according to an English transcript of the message released together with the recording.

The audio message was the first of Baghdadi since a video released in July, shortly after IS proclaimed a caliphate over parts of Iraq and Syria, of the militant supremo delivering a Friday sermon in the northern Iraqi city of Mosul.

The voice in Thursday's audio document appeared to match but there was no immediate confirmation of its authenticity from other sources.

The United States said that coalition aircraft launched strikes targeting Islamic State leaders near Mosul on Friday, setting off a flurry of speculation that Baghdadi was wounded.

Some reports pointed to another strike near Iraq's border with Syria, saying Baghdadi was wounded there instead.

In the recording, Baghdadi said that the US-led coalition of dozens of nations contributing to the military effort against IS was failing to defeat his group.

"Its march will not stop and it will continue to expand, by Allah's permission," he said. "The march of the mujahidin (holy warriors) will continue until they reach Rome." "And 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 said.