By Mick Krever, CNN

A prominent Syrian Sunni cleric on Monday condemned the ISIS killing of the American Peter Kassig and said that ISIS chief Abu Bakr al-Baghdadi “is going to hell.”

“We have to speak loud and very clear that Muslims and Islam have nothing to do with this,” Shaykh Muhammad al-Yaqoubi told CNN’s Christiane Amanpour.

“ISIS has no nationality. Its nationality is terror, savagery, and hatred.”

He expressed his “deepest condolences” to Kassig’s family, as well as to the families of the “many Syrians” who have been killed. (Kassig converted to Islam in captivity; his parents now refer to him as Abdul-Rahman.)

Al-Yaqoubi was an early critic of Syrian President Bashar al-Assad, and was forced to flee the country soon after the 2011 revolution began, having called on the president to step down.

He had been an imam and preacher at the regal Grand Umayyad Mosque of Damascus.

As the United States, Iraq, and their allies try to blunt ISIS’s advance militarily, thought leaders around the world are trying to grapple with why so many jihadis are flocking to Syria and Iraq, and how to stem the tide.

“There's no end unless we stop people from joining ISIS,” al-Yaqoubi said.

The imam has co-signed an open letter to al-Baghdadi that “refuted the ideological foundation of ISIS, banning every Muslim from joining it and making it clear that this is non-Islamic, anti-Islam, and we must put an end to it.”

He told Amanpour that Muslim scholars around the world must make clear that the ISIS leadership are falsely representing Islam.

“The ideology always existed. People from the beginning of Islam accused even the cousin of the Prophet of Islam – this is Ali, the fourth caliph – accused of being non-Muslim and they killed him.”

“They consider anyone who works for democracy as a heretic. They consider anyone who works with non-Muslims as an apostate. So there is no justification. They're just against the world. They're against Muslims; they’re against everyone.”

That is exactly the message al-Yaqoubi sees in the latest brutal ISIS video. In addition to claiming to have killed Kassig and a line of Syrian soldiers, the group for the first time claims that al-Baghdadi has accepted oaths of allegiance from across the region – Algeria to Yemen.

“They are challenging the world. They absolutely are hit, severely hit now and they are desperate and they want to fight to the end.”

“The problem with these people is they don't care if they are killed because they think they are martyrs – they're going to be in hell, of course. Every Muslim knows about this.”

ISIS, he said, “are not Syrians.”

“These people are bringing terror, savagery, and hatred from all around the world just in order to take revenge, they claim in the name of Islam, for the Syrian people who have been killed for three and a half years.”

“This is why I believe we need to topple the Assad regime, and this is the key point here – as long as the Assad regime continues its atrocities against the Syrian people, you’ll see more recruits and more savagery happening and taking place. We need to put an end to this and otherwise we might see just ISIS invading Damascus.”

“They're recruiting a lot of people. They have a huge army. They have an intelligence network. And all of that should be destroyed on the ground.”

Al-Yaqoubi recalled a Syria few are able to recognize amid the rubble and bloodshed of more than three years of civil war.

“Syria is an example of moderate Islam and tolerance – Jewish people, Christians, Shiites and all sects lived side by side. And we need this long history to come back.”

“Syrians are ready to heal their wounds and put their hands together against all odds and come out as a democratic nation, bringing freedom and dignity to the Syrian people and fighting terrorism wherever it exists in the world.”