US Secretary of State John Kerry launched a withering critique of the Islamic State Monday, blasting extremist brutality and insisting the world will not be intimidated in the battle against "barbarism."

Kerry's anti-IS broadside followed the jihadist group's claim that it had beheaded US aid worker Peter Kassig and 18 Syrian military personnel in a video that triggered worldwide revulsion.

The top US diplomat called for global partners to intensify efforts to crush the group that has seized large swathes of territory in Iraq and Syria, with nothing less than the fate of one of the world's most volatile regions at stake.

"This conflict is between civilization itself and barbarism," Kerry told a forum in Washington

"If we don't defeat ISIL there will be no viable future for the Middle East."

Kerry also defended a leading US role in efforts against IS, saying it had "become a threat to America's core interests."

US bombings of IS positions in Syria will continue, he said, adding the world will not bow to the jihadists' strategy of fear.

"Let us be clear: We are not intimidated, you are not intimidated, our friends and partners are not intimidated," Kerry said.

He stressed that failure to defeat IS at its roots could foment chaos for governments and communities a world away from the Middle East.

"Unless checked, this network could become a rallying point for the alienated and disaffected in every continent, spawning imitators and spurring individuals in far-flung places to commit stupid, destructive, suicidal acts," Kerry said.

While praising the broad nature of the coalition and noting how Arab and Gulf state have taken leading roles in anti-IS operation, Kerry also said Washington was "fully aware of the hazards associated with external military action."

"The US does not go in search of enemies in the Middle East," he said. "There are times, however -- and this is one -- when enemies come in search of us."

But he expressed hope that the horrors of IS were actually bringing often-adversarial Sunni and Shiite governments together in their resolve to defeat extremism.

"Ironically, we have found that our best recruiting tools (for the coalition) is ISIL itself," Kerry said, using another name for the group.

"ISIL is a coalition multiplier. Governments that can't agree on almost anything else agree on the imperative of confronting and defeating these terrorists."