The so-called Islamic State's media network published on Monday what it said was a video message from a man believed to be leader Abu Bakr al-Baghdadi, who had not been seen since 2014.
In the 18-minute video, a bearded man with Baghdadi's appearance said the so-called IS would seek revenge for the killing and imprisonment of its militants.
He added that the Sri Lanka Easter bombings were carried out in revenge of the so-called IS' losses in Syria.
"The battle for Baghouz is now over," the man said, referring to the last ISIS stronghold defeated by the Western-backed coalition on March 23.
Written script at the start of the video dated it to earlier in April. The authenticity and date of the recording could not be independently verified.
The video would be the first from Baghdadi since he was filmed in the Iraqi city of Mosul five years ago, though more recent speeches have been released as audio recordings.
Baghdadi, an Iraqi who declared himself the leader of the now-defunct "caliphate" in 2014, has frequently been reported killed or wounded.
But security sources have recently said he is thought to be hiding out in remote areas of Syria or Iraq.
Michael Kugelman, Deputy-Director of the Asia program at the Wilson Center, said the video was "perhaps part of an effort to demonstrate, after Sri Lanka, that ISIS has recovered and rebooted following the loss of its 'caliphate'. "
"A big worry is all those foreign fighters returning home from Syria and Iraq," the expert added.