By Sami Aboudi and Suleiman Al-Khalidi

DUBAI/AMMAN, Feb 4 (Reuters) - Muslim clerics widely condemned the burning to death of a Jordanian pilot by Islamic State, saying such a form of killing was considered despicable by Islam, no matter the context.

Islamic State militants released a video on Tuesday appearing to show captured pilot Mouath al-Kasaesbeh being burnt alive in a cage. Jordan, which has participated in a U.S.-led military campaign to bomb Islamic State positions, responded overnight by executing two al Qaeda convicts on death row.

Egypt's top Muslim authority, the 1,000 year old Al-Azhar university revered by Sunni Muslims around the world, issued a statement expressing "deep anger over the lowly terrorist act" by what it called a "Satanic, terrorist" group.

The Grand Sheikh of al-Azhar, Ahmed al-Tayeb, said the killers themselves deserved to be "killed, crucified or to have their limbs amputated."

Saudi cleric Salman al-Odah wrote on his Twitter account: "Burning is an abominable crime rejected by Islamic law regardless of its causes."

"It is rejected whether it falls on an individual or a group or a people. Only God tortures by fire," he added.

The Islamic State posted a religious edict on Twitter, which ruled that it is permissible in Islam to burn an infidel to death.

SHOCKING

But even clerics sympathetic to the jihadist cause said the act of burning a man alive and filming the killing would damage Islamic State, an al Qaeda offshoot which controls wide territory in Syria and Iraq, and is also known as ISIL or ISIS.

"This weakens the popularity of Islamic State because we look at Islam as a religion of mercy and tolerance. Even in the heat of battle, a prisoner of war is given good treatment," said Abu Sayaf, a Jordanian Salafist cleric also known as Mohamed al-Shalabi who spent almost ten years in Jordanian prisons for militant activity including a plot to attack U.S. troops.

"Even if the Islamic State says Mouath had bombed, and burnt and killed us and we punished him in the way he did to us, we say, OK but why film the video in this shocking way?" he told Reuters. "This method has turned society against them."

SITE, a U.S.-based monitoring service, quoted Abdullah bin Muhammad al-Muhaysini, whom it described as a Saudi jihadi, as saying on Twitter it would have been better if Kasaesbeh's captors had swapped him for "Muslim captives". His killing would make ordinary people sympathetic to Kasaesbeh, he said.

Still, some admirers of Islamic State cheered the killing. In a Twitter message, a user called Suhaib said: "To any pilot participating in the crusader coalition against the holy warriors - know that your plane might fall in the next mission. Sleep well!"

The killing was denounced in the Arab press. The pan-Arab al-Hayat newspaper published the report on its front page under the headline "Barbarity".

Saudi Arabia's Arabic daily al-Riyadh newspaper wrote that the Islamic state had "deepened its savagery and its bloody approach" by burning Kasaesbeh. (Reporting bySami Aboudi, Michael Georgy and Omar Fahmy in Cairo, Sylvia Westall in Beirut, Noah Browning in Ramallah, Editing by William Maclean)