Pressure mounts on U.K. prime minister after ISIS execution

Pressure on U.K. Prime Minister to fight ISIS... 02:21

Today in Paris, two dozen countries signed up to fight Islamic State of Iraq and Syria (ISIS), the Islamic terrorist army that is occupying much of Syria and Iraq. Secretary of State John Kerry said the U.S. would even be willing to talk to Iran about a role.

British PM David Cameron vows to "destroy" IS... 03:06

The fight has taken on more urgency with the beheading of a third westerner a British citizen this time.

The CIA estimates the ISIS now has up to 31,000 fighters. Some countries have to be convinced to join the U.S.-led campaign against them, but Britain had no choice.

David Haines, the aid worker whose execution video was posted by ISIS this weekend, is British and his presumed executioner - the same man who appeared with the American journalists Steven Sotloff and James Foley - is also believed to be from the U.K.

Also, the Security services in the U.K. believe that 500 British men - and even some women - have joined ISIS' ranks.

Combating ISIS: What are U.S. allies willing ... 02:56

Prime Minister David Cameron said the country must confront the ISIS menace.

"They claim to do this in the name of Islam," said Cameron. "That is nonsense. Islam is a religion of peace. They are not Muslims, they are monsters.

But Cameron stopped short of authorizing airstrikes. He has his hands full this week, trying to prevent the country from breaking up in a Scottish independence referendum on Thursday and it's not clear if public opinion would support a British bombing campaign.

But the fate of Alan Henning may shortly force his hand.

He's a Manchester taxi driver who joined a group of Muslim friends on an aid convoy to Syria just before Christmas last year. Henning made it over the border and was immediately taken prisoner.

Now ISIS says he will be the next to die and as a horrified world has learned recently - they're deadly serious.

The latest poll shows a majority of Britons are already in favour of the US airstrikes against ISIS, and analysts think another beheading would shift public opinion to back Britain joining in.