Top US diplomat John Kerry revealed Wednesday he was working to forge a global coalition to fight the "medieval savagery" of Islamic militants terrorizing a swathe of Syria and Iraq and blamed for killing two journalists.

As President Barack Obama called for an international front against the group known as the Islamic State (IS), US officials also revealed that two videos showing the beheadings of Steven Sotloff and James Foley were not filmed at the same time.

"The real face of Islam is not what we saw yesterday, when the world bore witness again to the unfathomable brutality of ISIL terrorist murders," the US secretary of state told a ceremony to honor the new special US envoy to Muslim communities.

Kerry called the 31-year-old Sotloff "a driven and courageous journalist," saying he was killed by a "coward hiding behind a mask."

Having worked to try to bring Sotloff home safely, Kerry said his murder so soon on the heels of Foley's beheading last month was "a punch to the gut."

The United States would hunt down Sotloff's killers, Kerry vowed, as Obama said during a visit to Estonia that "if we are joined by the international community, we can continue to shrink ISIL's sphere of influence, its effectiveness, its financing, its military capabilities."

"The face of Islam is not the butchers that killed Steven Sotloff. That's ISIL. The face of Islam is not the nihilists who know only how to destroy not to build," Kerry said.

"It's not masked cowards, whose actions are an ugly insult to the peaceful religion that they violate every single day with their barbarity," he insisted. "The real face of Islam is a peaceful religion, based on the dignity of all human beings."

As NATO leaders prepare to gather in Wales on Thursday, Kerry said he would travel further afield in the coming days seeking to win support for the coalition.

- Telephone diplomacy -

He pledged to be guided by the example set by one of his predecessors James Baker, who pieced together the front against late Iraqi leader Saddam Hussein in the 1991 Operation Desert Storm.

It was "the gold standard by which modern coalition-building is judged and which I will personally use as I go out in the next days to work on the ISIL issue," Kerry said at a seperate event.

In a rush of telephone diplomacy, he spoke Tuesday with counterparts from Australia, Italy, Jordan, Qatar, and the United Arab Emirates.

The US was "reaching out to a range of countries that have a desire to be a part of this coalition," State Department spokeswoman Jen Psaki said.

"There are already a number of countries that may not be in the Arab world or aren't in Europe who are already contributing resources and offering assistance to fight ISIL and to address the humanitarian situation Iraq," she added.

"Destroying and degrading ISIL" would be the goal of the coalition as well as seeking to bring about stability in both Iraq and Syria.

US intelligence officials are poring over the videos of the journalists murders for clues to help them in the hunt for IS leaders and Psaki revealed that they were sure that the two videos were not shot at the same time.

Kerry also paid tribute to Sotloff, saying his "reporting was as empathetic as his killers are evil" and comparing him to legendary 20th century war correspondent Martha Gellhorn.

"He focused on the stories of average people trapped in war, and documented their day-in and day-out struggle for dignity," Kerry said.