The so-called Islamic State's Al Furqan media network has published what it said was a video message from its leader Abu Bakr al-Baghdadi in which he said the group would seek revenge for the killing and imprisonment of its militants.

In the video, a bearded man with Baghdadi's appearance sitting cross-legged on the floor gives an 18-minute address. Several aides with their faces covered are shown listening.

Written script at the start of the video dated the video to earlier in April. The authenticity and date of the recording could not be independently verified.

This evening, the United States vowed that it would track down and defeat surviving leaders of the IS movement.

Baghdadi
A file photo of Abu Bakr al-Baghdadi from 2014

The US-led coalition against the group will fight across the world to "ensure an enduring defeat of these terrorists and that any leaders who remain are delivered the justice that they deserve," a US State Department spokesman said.

In the video, the man said to be Baghdadi also referred in the past tense to the fight for Baghouz, IS's final bastion in eastern Syria, which ended last month.

"The battle for Baghouz is over," the man said.

But he insisted that IS's operations against the west were part of a "long battle," and that IS would "take revenge" on members who had been killed.

"There will be more to come after this battle," he said.

Baghdadi, 47, had a long grey beard that appeared dyed with henna and spoke slowly, often pausing for several seconds in the middle of his sentences.

He appeared for the first and last time in public in Mosul in 2014, where he declared an Islamic "caliphate" in the swathes of territory IS then held in Syria and Iraq.

He was reported killed or injured multiple times since then.