US President Barack H Obama and UK Prime Minister David Cameron intend to build an international coalition against ISIS, the terror group that has conquered large portions of Iraq and Syria. In the wake of the revelation of a second video showing the beheading of a Western journalist by ISIS this week, Obama and Cameron intend to lobby for the coalition's assembly at a NATO summit Thursday.

“We will not be intimidated," Obama said in Estonia on his way to the summit. "Their horrific acts only unite us and stiffen our resolve to take the fight against these terrorists. And those who make the mistake of harming Americans will learn that we will not forget and that our reach is long and that justice will be served. It is going to take time for us to form the regional coalition that's going to be required so that we can reach out to Sunni tribes in some of the areas that Isis has occupied, and make sure that we have allies on the ground in combination with the air strikes that we've already conducted,” Obama said.

After releasing the video of the beheading of Steven Sotloff this week, which followed the beheading of James Foley several weeks ago, ISIS claimed that the next victim will be British. That victim is likely to be David Cawthorne Haines of the UK with death as well, who is threatened in the Sotloff beheading video as the next one to be killed.

Cameron said that the UK could join the US in its airstrikes against ISIS positions in Iraq. “We should do what we can to help those on the ground who want to build an Iraq for all Iraqis: Sunnis, Shias and Kurds. We've helped already with aid,” Cameron said. “We've helped with other military assets, and we'll always ask ourselves what is in our national interest. Not ruling things out, but going forward in a deliberate, sensible, resolute way."

France is likely to be on board with such a coalition. Speaking Thursday, French President Francois Hollande said that “there is a great deal of importance attached to humanitarian missions and attempts to solve this politically, but it is possible that a military response, in accordance with international law, will be necessary.”

Speaking Wednesday, U.S. Vice President Joe Biden issued a sharp warning to the Islamic State group, saying after the United States is done grieving the death of two American journalists, their killers will have to answer for their actions. "They should know we will follow them to the gates of hell until they are brought to justice," he forcefully told an audience at an event on the New Hampshire-Maine border, according to CNN . "Because hell is where they will reside. Hell is where they will reside."

Meanwhile on Wednesday, Obama vowed that the United States would not be intimidated by the beheading of Sotloff, but acknowledged the fight against the IS would take time. Obama pledged that justice would be done to the IS murderers who brutally beheaded Sotloff, who was also an Israeli citizen, wherever they hid and however long it took. Obama said the whole world had been repulsed by the barbarism of Sotloff's murder, but "we will not be intimidated. Those who make the mistake of harming Americans will learn that we will not forget and that our reach is long and that justice will be served."