Perhaps most ominously from a British point of view, in the video which claimed to show the beheading of Steven Sotloff, there is also now an additional threat. That it [Islamic State] will kill - behead - a British hostage, who it allegedly holds in its hands. There is real concern now for the British security services about whether the Islamic State might carry out this latest threat.
A small but very revealing and significant shift is underway. The White House is no longer saying no US troops on the ground. Rather no US troops on the ground in a combat role, recognising the reality that some kind of rescue mission is going to happen and very quickly.
Obviously one option is a humanitarian corridor. I think more likely is some kind of massive airlift, simply because of the urgency of the situation and the judgement that many of the displaced people are too weak to walk, and that a corridor would be very difficult and problematic to try to secure.
Even an airlift would be challenging because clearly Islamic State militants would regard killing or capturing any US soldiers or airmen as a major propaganda coup. It is clear that even in terms of force protection, this could slip into a combat situation. For this President that would be a massive reversal, which is why tonight the White House is still saying no final decision has yet been made.
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Just dropping food and supplies on a remote mountain in northwestern Iraq doesn't begin to solve the problem - where do these tens of thousands of Yazidis and Christians go? If you want to create a humanitarian corridor or a safe haven it inevitably involves ground troops and that is something the White House has emphatically ruled out.
It is fair to say there is an increasing likelihood of not just air drops but of air strikes and then America will be left to hope that Kurdish and Iraqi government forces come to the rescue on the ground. One thing is for sure: the White House sees doing nothing - effectively being a spectator to this tragedy - as the worst option of all.
The equation has changed for US foreign policy in Iraq because of the gravity of the humanitarian situation unfolding at Mount Sinjar.
President Obama, we all know, is a reluctant interventionist but with up to 40,000 internally displaced people huddled on a mountain top and surrounded by Isis militants threatening to slaughter them, obviously the moral pressure to act is immense.
One option is simply to drop food and supplies to them but it raises the question 'Is that sufficient?' given what might become of them. So then do ground troops become deployed in order to create some sort of humanitarian corridor? Does America decide to strike these militants to try to protect these tens of thousands of refugees?
You can see how very quickly America could get dragged back into the Iraq War, which of course it is deeply reluctant to do. But, on the other hand, doing nothing and being a spectator might simply not be tenable.
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By ITV News Washington Correspondent Robert Moore
I think the Americans see a perfect storm brewing in the Middle East - really a war within a war - as various Jihadi groups fight each other for relevance and supremacy.
The concern in the US for counter-terrorism groups is that either Isis or its rivals in terms of al-Qaeda affiliates might seek to gain notoriety and new followers by attacking the US home land.
I don't believe there's a specific threat the US intelligence has intercepted.
But there is alarm here that some of these groups have gained deep technical knowledge of bomb-making as well as access to European followers - those are individuals who may not need a US visa and who with relative ease could get onboard a US-bound airliner.
I think it's clear that these stunning advances of Isis in Iraq and Syria is certainly ringing alarm bells here in Washington.
There's a recognition that Isis is a ruthless enemy, that it has access to tens of millions of dollars and potentially continuing revenue through oil resources, and that it might also want to make a global statement of its ambitions.
You throw into the mix the fact that there may be other al-Qaida affiliates, rivals to Isis that are seeking to show their continuing relevance and credibility on the Jihadist stage and it's pretty clear why the Department of Homeland Security are really very concerned tonight.