Police in Britain are "urgently investigating" a video apparently showing a British Islamic State (IS) group fighter railing against Prime Minister David Cameron over air strikes in Iraq, Downing Street said Saturday.

Speaking with a British accent and with his face uncovered, the man, identified by the SITE intelligence group as Abu Saeed al-Britani, condemns Cameron and the United States for launching air strikes on IS jihadists. He also urges British Muslims to launch attacks in Britain.

The family of Briton Alan Henning, whose beheading has been claimed by the Islamic State (IS) group, said Saturday they were "numb with grief" over the murder of a "decent, caring" man.

Henning's wife Barbara and his two children said they had hoped never to receive news of his killing but learned that he was dead after IS released a graphic execution video late Friday.

British fighter jets on Tuesday bombed an artillery post and an armed truck used by the Islamic State group in Iraq in the Royal Air Force's first strikes in the U.S.-led air campaign.

The defense ministry said two Tornado jets hit the post with a Brimstone missile used against tanks and the vehicle with a 500-pound (230 kilogram) Paveway IV laser-guided bomb.

The British parliament will hold an extraordinary session on Friday to debate and vote on joining in U.S.-led airstrikes against the Islamic State group in Iraq, Prime Minister David Cameron said on Wednesday.

Speaking after talks with Iraqi Prime Minister Haidar Al-Abadi in New York, Cameron said he was "confident" that members of the House of Commons would authorize Britain to join a coalition conducting air strikes in Iraq.

Britain cannot opt out of a battle against Islamic State jihadists, Prime Minister David Cameron said on Tuesday, as newspapers reported he was considering joining air strikes targeting the group.

"This is a fight you cannot opt out of. These people want to kill us," Cameron told an interview with NBC news, saying that the militants had planned attacks in Europe and elsewhere.

Britain's main parties squabbled on Sunday over how to deliver reform after Scotland voted "No" to independence, after Prime Minister David Cameron insisted any new powers for the Scots must see similar autonomy for the English.

The united front of the referendum campaign, when the Conservatives, Labor and Liberal Democrats promised greater devolution for Scots if they stayed in the United Kingdom, is cracking over concerns of the wider implications.

Baluch separatist leaders on Friday called on Pakistan to follow in Britain's footsteps by holding a referendum similar to Scotland's on granting independence to the insurgency-wracked province.

Scots rejected independence in a vote that left the centuries-old United Kingdom intact despite a surge in nationalist support in the final fortnight of the campaign.

The Islamic State group released a video Thursday of a British freelance photojournalist, John Cantlie, in which he says he is being held captive.

In the video posted on YouTube, Cantlie, wearing an orange jumpsuit, speaks to the camera in the style of a news report and promises to reveal in a series of programs the "truth" about the jihadist group that has seized parts of Iraq and Syria.

Thousands gathered in London on Monday to urge Scottish voters to reject independence and stay in the United Kingdom in a landmark referendum in three days' time.

Demonstrators in the rally in central Trafalgar Square waved Union Jack flags and held up signs reading "Let's stay together" and "Scotland we love you, don't go."

British Prime Minister David Cameron on Monday made a trip to Scotland to urge Scots to vote against independence and warned them of dire consequences otherwise in his most forceful speech yet ahead of this week's historic referendum.

Cameron was speaking in Aberdeen on what could be his last visit to Scotland before the vote on Thursday and promised to give Scotland sweeping new powers in the event of a "No" result against independence.