Although Washington and Tehran are currently in a standoff after tit-for-tat military actions over the past two weeks, the king told the European Parliament that the danger has not passed.

"What if next time neither side steps back from the brink, dragging us all towards untold chaos? An all-out war jeopardises the stability of the entire region," he said.

"What's more, it risks massive disruptions of the entire global economy including markets, but threatens a resurgence of terrorism across the world."

The alarm was among a raft of other warnings by King Abdullah, a pro-Western leader whose country is haven of relative stability in a Middle East roiled by proxy conflicts, sectarian violence and competition between powers inside and outside the region.

Urging greater leadership and "patience" to address the tensions, Abdullah expressed concern about developments in Syria and Iraq.

"What if Syria remains hostage to global rivalries and spirals back into civil conflict? What if we see a reemergence of ISIS and Syria becomes a staging ground for attacks against the rest of the world?" he asked.

Turmoil in Iraq, he said, risked tipping that country into a cycle of "recovery and relapse -- or, worse yet, conflict".

He also homed in on Libya, one of the biggest foreign policy issues facing the EU along with Iran.

"What if Libya collapses into an all-out war, and ultimately a failed state? What if Libya is the new Syria, just much closer to the continent you all call home?" he asked, saying such scenarios needed to be addressed now to prevent them becoming reality.

The Jordanian monarch, who carries the hereditary title of "custodian" of holy Muslim and Christian sites in Jerusalem, also stressed to MEPs that Israel was trying to "impose an unthinkable solution" over Palestinians as hopes fade for a two-state solution backed by the international community.

He said Israel's construction of settlements in occupied Palestinian territory and "disregard of international law" could be summed up as "one state turning its back on its neighbourhood, perpetuating divisions among peoples and faiths worldwide".