Officials Hopeful ISIS Leader's New Audio and Sightings Mean He's Cornered Abu Bakr al-Baghdadi suspected to be in Mosul, Iraq.

 -- As the top leader of ISIS, Abu Bakr al-Baghdadi, released a new audio speech today calling on his fighters to "decimate" Iraqi military forces closing in on his stronghold Mosul, counter-terrorism officials say unconfirmed sightings there of the so-called "caliph" have them increasingly hopeful he's "surrounded."

U.S. military analysts strongly suspect that al-Baghdadi is holed up in Mosul, Iraq's second-largest city, now facing a siege by Iraqi and Kurdish forces and backed by American airpower and special operations forces. One knowledgeable counter-terrorism official said human intelligence in Mosul has supposedly spotted the terrorist leader, who has otherwise only appeared in public on video once in July 2014.

"There have been sightings but no proof it was actually him," a counter-terrorism official told ABC News.

Al-Baghdadi is known to have spent more time in Mosul than in the ISIS de facto capital of Raqqa, Syria over the past two years, and there is no evidence he is in Syria now, officials said. The U.S. Joint Special Operations Command (JSOC) has never had real-time intelligence on his whereabouts in order to kill him.

Past claims by Iraqi officials that the ISIS leader was wounded in airstrikes have been regarded as false by the U.S. military, numerous counter-terrorism sources have told ABC News.

Al-Baghdadi in his new speech renewed past efforts to motivate his fellow Sunni Muslims by claiming enemy Shi'a Muslims will destroy them, urging supporters to "start your actions" in Turkey and Saudi Arabia, according to an English translation of the speech by the SITE Intelligence Group.

Some civilian experts viewed the 30-minute audio -- in an unusually fast-tempo, crisp Arabic -- as a desperate act to reassure his forces and urge them to carry on even if he or other ISIS leaders are "martyred."

"His message to his people is that all of this has been foreseen so don't panic," ISIS expert and author J.M. Berger told ABC News.

The audio itself was clearly produced recently in response to the Mosul offensive, added Berger, who co-wrote the book, "ISIS: The State of Terror."

"The speech is attempting to downplay reversals suffered by ISIS, particularly as the battle for Mosul ramps up," said Phillip Smyth, who researches Shi'a militant groups at the University of Maryland. "But [al-Baghdadi] also attempts to project the image that the caliphate is strong... even while under extensive military pressure."

A senior counter-terrorism official said the change in al-Baghdadi's once-optimistic message is notable.

If senior military analysts are right that al-Baghdadi is hunkered down in Mosul with less than 6,000 fighters remaining, and possibly surrounded by loyalists strapped with explosive suicide belts to kill him if enemy forces get too near, then his audio may be his public last stand.

"He's trapped in the city. It's totally surrounded," the first counter-terrorism official said.