US Secretary of State John Kerry says the arrests in Australia of a group allegedly plotting "spectacular, horrendous, grotesque atrocities" shows the threat Islamic State jihadists pose worldwide.
After rampaging through Syria and Iraq and gaining territory, and access to funding dwarfing what al-Qaeda was able to secure, Kerry said Islamic State group was looking for targets outside the Middle East.
"Now, they are plotting externally," Kerry said in an interview on America's MSNBC on Monday.
"In Australia, they just arrested a group of people who were alleged to be about to do spectacular, horrendous, grotesque atrocities".
Kerry's comments came as Australian Prime Minister Tony Abbott prepared to fly to New York for two days of top-level talks at the United Nations.
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US President Barack Obama, Abbott and other world leaders will be in Manhattan where the plan to take on the Islamic State group will top the agenda.
Kerry said there are more than 100 fighters from America fighting for the terror group in Iraq and Syria.
He described the Islamic State jihadists as "one of the most dangerous groups that I have seen in my time in public life".
"They have passports," Kerry said.
"They can come back here.
"That's why foreign fighters are such a focus, and President Obama will chair a UN Security Council meeting this week to deal with foreign fighters."
About 800 police conducted pre-dawn raids across Sydney and Brisbane last week, with authorities alleging a terror plot involving the planned beheading of a random person.
