The Islamic State (IS) group, which has sown a trail of terror across Syria and Iraq, is neither a state nor justified by Islam, Egypt's top Muslim cleric said Monday.

"We have already asserted that we should never use the term Islamic State for ISIS," Egypt's Grand Mufti Sheikh Shawky Allam said at a public forum in Singapore.

"It is not a state in the first place, neither is it justified by any Islamic justification," he told the packed gathering, which was organised by the city-state's Islamic Religious Council.

"The establishment of any Islamic state will need to go to its roots, its basis and foundations," he said through an interpreter when asked if the formation of the group, also known as the Islamic State of Iraq and Syria (ISIS), was in line with Islamic law.

Since the group emerged in its current form in 2013, its militants have captured large swathes of territory in both Syria and Iraq.

It has declared an Islamic "caliphate" in territory under its control, and gained a reputation for brutality, including torture and beheading of its captives.

In its latest act of brutality, the group said Sunday it had executed one of two Japanese hostages it has been holding, apparently by beheading him.

Sheikh Allam said the group was misrepresenting Islam.

"The main intention of Allah is certainly not the destruction, killing or war," he said.

"What we seek is stability. If these people are there to only cause destruction, and you say you do it in the name of Islam, certainly this is not what Islam is about."

Sheikh Allam also said the publication of Prophet Mohammed cartoons by the French satirical magazine Charlie Hebdo "affected our emotions deeply" but stressed that "we have to deal with this issue in a rational manner".

Two Islamist gunmen this month attacked the Paris office of the magazine, leaving 12 people dead, including some of Charlie Hebdo's top cartoonists. Five more people were killed in related attacks by a separate gunman.

"When someone has committed a crime in any country, there are ways to ensure this matter is raised through proper channels," the cleric said.

"Anything we do must be just -- that is the basis of Islam."