David Cameron: "We should not turn away from what needs to be done"
Parliament is to be recalled on Friday to discuss the UK's possible involvement in air strikes against Islamic State (IS) in Iraq.
Prime Minister David Cameron said MPs should respond to the Iraqi government's request for help.
He added that the UK "should not turn away from what needs to be done".
The Liberal Democrats are backing air strikes in Iraq and Labour leader Ed Miliband has confirmed his support, saying the UK cannot "opt out".
Meanwhile, the Foreign Office has said it is "aware" of reports that a British national has died in Syria, but it has "no further information at this moment".
IS - also known as Isil - has taken control of large areas of Iraq and Syria in recent months and seized several Western hostages.
It has threatened to kill British aid volunteer Alan Henning, having released footage of the killing of another British man, David Haines, earlier this month.
Mr Cameron, who is in the US, tweeted: "I have requested that Parliament be recalled to debate the UK response to the Iraqi Govt's request for support against Isil. The Speaker has accepted my request to recall Parliament on Friday."
He later said: "What we are doing is legal and it is right. It does not involve British combat troops on the ground."
IS militants have taken over large areas of Iraq and Syria
He added that "when we are threatened in this way, we should not turn away from what needs to be done".
"I'm confident we will get this through on an all-party basis," Mr Cameron said.
"If there was a question of taking action against Isil in Syria, it would be a separate parliamentary debate. I want to be very clear about that."
Deputy Prime Minister and Liberal Democrat leader Nick Clegg told the BBC his party would support air strikes in Iraq.
He said they were "legal" and the UK would be "part of a much bigger coalition, a whole array of countries, crucially including a number of Arab countries which deprives Isil of the ability to somehow portray it as a west verses the rest crusade".
How recalls happen - by Shelley Phelps, BBC political research unit
If government ministers want the Commons to meet when it is in recess they must ask the Speaker to make a decision on whether or not to recall MPs to Parliament.
If the Speaker agrees that the request is in the public interest, he will then decide what time and day the Commons should meet and send out a notice to MPs. This can be issued as late as the day before the recall.
MPs do not have the power to ask the Speaker for the Commons to be recalled.
The Commons was last recalled on 29 August 2013 to debate a motion on Syria. This was the 27th recall during a recess since 1948.
Mr Cameron will open Friday's debate, due to start at 10:30 BST, and Mr Clegg will close it, with a vote expected at 17:30 BST. The cabinet is meeting to discuss the situation on Thursday.
Mr Miliband said: "We will be supporting the government on air strikes by Britain in Iraq.
"Isil is a murderous organisation. It's a murderous organisation against people of all nationalities and all religions. It is a threat directly to the stability of that region, and indeed to Britain because it can be a breeding ground for terrorism. I think is those circumstances we can't turn away from that threat."
The US and five countries from the Gulf and Middle East have begun bombing IS in Syria.
Tens of thousands of Syrian Kurds have fled to Turkey
Mr Cameron lost a Commons vote in August last year over proposed air strikes in response to Syria's President Bashar al-Assad's alleged use of chemical weapons.
BBC political editor Nick Robinson said that, because of Labour concerns, Parliament would be asked to approve UK involvement in military action in Iraq, but not in Syria.
The UK is already providing arms and surveillance support to Kurdish forces who are battling IS.