King Abdullah of Saudi Arabia has warned that the West will be the next target of the jihadists sweeping through Syria and Iraq, unless there is "rapid" action.
"If we ignore them, I am sure they will reach Europe in a month and America in another month," he said in remarks quoted on Saturday by Asharq al-Awsat daily and Saudi-backed Al-Arabiya television station.
"Terrorism knows no border and its danger could affect several countries outside the Middle East," said the king who was speaking at a welcoming ceremony on Friday for new ambassadors, including a new envoy from Saudi ally the United States.
The Islamic State (IS) jihadist group has prompted widespread concern as it advances in both Syria and Iraq, killing hundreds of people, including in gruesome beheadings and mass executions.
Lack of action would be "unacceptable" in the face of the phenomenon, King Abdullah said.
"You see how they (jihadists) carry out beheadings and make children show the severed heads in the street," he said, condemning the "cruelty" of such acts.
"It is no secret to you, what they have done and what they have yet to do. I ask you to transmit this message to your leaders: 'Fight terrorism with force, reason and (necessary) speed'."
President Barack Obama has yet to decide whether the United States should launch raids against positions held by the Islamic State jihadist group in Syria to follow US air strikes on IS activities in Iraq.
US Secretary of State John Kerry called Friday for a global coalition to combat Islamic State fighters' "genocidal agenda".
Writing in the New York Times, Kerry said he and Defense Secretary Chuck Hagel will meet European counterparts on the sidelines of a NATO summit in Wales next week, to enlist assistance.
They will then travel on to the Middle East to build support "among the countries that are most directly threatened".
"With a united response led by the United States and the broadest possible coalition of nations, the cancer of ISIS will not be allowed to spread to other countries," Kerry said in Friday's op-ed piece.
Asharq Al-Awsat said the king urged other countries to join the UN Counter-Terrorism Centre, set up in 2011 to respond to new threats, and to which Saudi Arabia has made a grant of $100 million.