What is the World Going to do About ISIS?

After the beheading of another Western captive by ISIS, an international conference convened Monday in Paris to talk about how to tackle the threat posed by the Islamic extremist group.

The Sunni jihadist group underlined its barbaric credentials over the weekend, posting a video showing the beheading of British aid worker David Haines and threatening the life of another hostage from the United Kingdom.

It was the third videotaped killing of a Western hostage released in less than a month.

The latest killing, ISIS said, was "a message to the allies of America" - a direct challenge to the United States.

President Barack Obama announced last week that the United States would lead "a broad coalition to roll back this terrorist threat," and that U.S. airstrikes against ISIS would expand from Iraq into Syria.

The United States has said nearly 40 nations have agreed to contribute to the fight against ISIS, which has seized control of large areas of northern Iraq and Syria. But it remains unclear exactly which countries are on that list and what roles they'll play.

The Paris conference Monday is being hosted by French President Francois Hollande and his Iraqi counterpart Fuad Masum.

At the start of the event, Hollande said there was "no time to lose" in international efforts against ISIS.

See more on this developing story on CNN.com