Saudi Arabia's top cleric has urged confrontation with the "oppressive" Islamic State (IS) group if it fights Muslims after seizing swathes of Iraq and Syria, media reports said Sunday.

"This group is aggressive and oppressive. It sheds blood," Al-Eqtisadiah daily quoted Grand Mufti Sheikh Abdul Aziz al-Sheikh as saying.

"If they fight Muslims, then Muslims must fight them to rid people and religion of their evil and harm," he said in a response to a request from an Iraqi for a fatwa or edict on fighting IS.

"They have been killing ever since they began their fight. Their killing is filled with mutilations and hideousness that distort [the image of] Muslims," Sheikh said.

His remarks come as the United States seeks to build a broad-based international coalition to fight the IS, which has carved out a stronghold in large areas of Syria and Iraq.

The United States last month launched air strikes against the militant group in Iraq, in support of government forces and allied tribesmen, as well as Kurdish peshmerga fighters in the north.

Last month, the desert kingdom's highest religious authority branded Al-Qaeda and IS jihadists Islam's "number one enemy", and warned Muslim youths to steer clear of "calls for jihad" issued on "perverted" grounds.

On 29 June King Abdullah said “we will not allow a handful of terrorists, using Islam for personal aims, to terrify Muslims or undermine our country and its inhabitants."

Analysts said Gulf States have previously avoided confronting domestic funders of the group and are coming under increasing US pressure to confront the group.

“ISIS [IS’ former name] could not have come about without the Gulf States turning a blind eye to funding,” Toby Matthiesen, a Gulf expert at the University of Cambridge, told The Guardian last week. “It’s this ad hoc mentality where they do something and don’t think of the consequences. They felt they occupied the moral high ground.”

“They wanted Assad to go but now are confronted with people who want to take over Mecca and Medina on the basis of Salafi Wahhabi ideology. Monarchs don’t fit into that. Potentially there are a lot of ISIS supporters in Saudi. It is really tricky situation for them and they are under heavy pressure from the west to show they are fighting ISIS.”

IS has faced increasing condemnation from across the region, including damnation by public figures linked with Al-Qaeda.

Abu Qatada, who is facing terror charges in Jordan after being deported from the UK, denounced the group’s apparent beheading of two American journalists, calling the IS a “killing machine”.

Abu Qatada was once described by a Spanish judge as the right-hand man in Europe of Al-Qaeda’s founder Osama bin Laden.

He was deported from Britain to Jordan in July 2013, after a 10-year legal battle, to face two separate trials.

In June he was acquitted of plotting a 1999 attack on the American school in Amman but is still being tried on another terror charge of conspiring to attack tourists in Jordan during millennium celebrations.

Speaking to reporters from the dock at an Amman courthouse Sunday, Abu Qatada repeated his condemnation of the IS.

Asked about the beheading claimed by IS of American journalists James Foley and Steven Sotloff, he said: "Journalists should not be killed because they are messengers of the truth."

He lambasted IS, branding it "a killing and demolition machine" and likened its fighters to "dogs of hellfire".

Nevertheless Abu Qatada said he opposed plans by the US to set up an international coalition to destroy the group, saying: "I am against any coalition opposed to any Muslim."

The court, meanwhile, postponed his trial to September 24.

In July, Abu Qatada and another Jordanian linked to Al-Qaeda denounced the IS for declaring an Islamic caliphate in territories under its control in Iraq and Syria.

"The announcement of a caliphate by the IS is void and meaningless because it was not approved by jihadists in other parts of the world," Abu Qatada wrote in a 21-page document posted online.

Abu Mohammed al-Maqdessi, once mentor to Iraq's now slain Al-Qaeda leader Abu Musab al-Zarqawi, said IS must "reform, repent and stop killing Muslims and distorting religion".