ISIS: Timeline of hostage killings
The murder of aid worker David Haines is the third such killing to be filmed and released by the Islamic State (IS). Here is a timeline of events, starting with the days leading up to the death of the first hostage, US journalist James Foley.
Summer 2014: A rescue mission fails to free Mr Foley from a secret location where he is being held by the Islamic State (IS). The American freelance journalist, 40, was captured two years earlier.
August 13: Mr Foley's family receive a message that he will be executed.
August 19: al-Furqan Media, which is controlled by the terror group, releases a video appearing to show Mr Foley's killing. His death apparently happened at some time in the previous week. Social media bosses are forced to act swiftly in banning both the video and still images of the beheading, while concerns mount for fellow reporter and captive Steven Sotloff, who is seen pleading for the US to stop air strikes in Iraq at the end of the video.
August 20: David Cameron cuts short a family holiday in Cornwall to hold meetings in Downing Street over the "shocking and depraved" murder. Meanwhile, US President Barack Obama remains defiant over his country's war on terror. He says: "We will be vigilant and we will be relentless."
August 21: The killer is reported to be a British man who goes under the moniker "Jihadi John".
August 24: The Government comes under pressure to strip British jihadists of their citizenship as reports emerged that MI5 and MI6 have now identified the apparently British killer behind the horrific murder.
August 25: Shadow Home Office minister David Hanson repeats Labour's call for the return of control orders - which bans terror suspects from areas and restricts certain freedoms. They were previously abolished by Home Secretary Theresa May in favour of Terrorism Prevention and Investigation Measures (Tpims).
August 26: President Obama states his determination to quash IS, previously known as ISIL, but says a response to Mr Foley's death may take some time. He says: "Rooting out a cancer like ISIL won't be easy and it won't be quick."
August 27: General Sir Richard Shirreff, who stepped down from his post as Nato deputy supreme commander earlier this year, says nothing should be ruled out in the fight against IS in Iraq and Syria.
September 2: Mr Cameron condemns the "disgusting and despicable" video apparently showing Mr Sotloff wearing an orange jumpsuit and being beheaded by Islamic State militants. The alleged killer speaks with a British accent, prompting suggestions he is the man who murdered Mr Foley. It emerges that a British hostage, now known to be aid worker David Haines, is also under threat.
September 3: Following a meeting of the Government's Cobra emergency committee, Mr Cameron tells MPs in the House of Commons that Britain "will never give in to terrorism". He insists Britain will refuse to pay ransoms to terrorists in return for the release of hostages.
September 4: At the start of a Nato summit in Wales, leaders of the Western alliance show a united front in response to terror threats. Mr Cameron and Mr Obama insist they will not weaken in the face of threats from IS terrorists. The Prime Minister refuses to rule out joining the US in air strikes on IS. He says Britain must "use everything we have in our armoury" to squash the terrorists out of existence.
September 5: Foreign Secretary Philip Hammond joins Mr Cameron in saying the Government will not hold back on launching air strikes, and says threats on Mr Haines' life by his captors will not deter the UK from taking action. The Nato summit ends with a pledge to "degrade and defeat" IS militants.
September 7: On his return from Wales, Mr Obama says he is to begin laying out a strategy to defeat IS militants. In England, chancellor George Osborne says the Government "reserves the right to take action" to stem any immediate humanitarian crisis on the ground in Iraq, although he says the Government is not at yet at the stage of agreeing targeted air strikes.
September 8: Mr Cameron tells MPs they will have an opportunity to vote on whether to join the US in any military action.
September 9: Defence Secretary Michael Fallon confirms Britain will supply £1.6 million of machine guns and ammunition to Kurdish forces fighting IS militants in northern Iraq as part of a burgeoning international effort to drive back the jihadists .
September 10: Mr Obama authorises air strikes inside Syria for the first time as well as an expansion of strikes in Iraq. Earlier, during a visit to Baghdad, US secretary of state John Kerry says that neither his country nor the rest of the world would stand by and watch the IS militant group spread its evil.
September 11: Mr Obama's plans are welcomed by Mr Cameron, though Downing Street again insists the UK is "not at the stage" of joining them in military action.
September 12: In a rare move, Europe minister David Lidington is summoned to Parliament to explain the Government's stance on tackling terror. He insists Mr Hammond had deliberately not ruled anything out when he answered reporters' questions about the UK's plans.
September 13: A video showing the murder of British aid worker Mr Haines is released. The video, which involves the 44-year-old saying Mr Cameron and Tony Blair should be held responsible for his death, is widely condemned. It comes hours after Mr Haines' family issue a plea to his captors to contact them. The clip also includes a threat to kill a second British hostage.
September 14: Mr Cameron calls an emergency meeting of its Cobra committee in the wake of the murder.