Islamic State has urged its followers to attack citizens of the United States and other countries that have joined a coalition aimed at wiping out the radical group.
A spokesman for the militants claimed the intervention by the US-led coalition would be "broken and defeated", according to a transcript of an audio recording in Arabic.
Washington is building an international coalition to combat Islamic State, which has taken control of large parts of Iraq and Syria. Countries including France, Canada and Australia have pledged their support.
The US has said that other countries would be willing to join the coalition if it proceeds with air strikes against the group in Syria in addition to those made in Iraq by American and French jets.
A former director of British special forces has said the West "should not underestimate" the threat Islamic State militants pose.
Lieutenant General Sir Graeme Lamb, who served in Iraq and Afghanistan, told BBC Radio 4 it was important to contain IS as they presented themselves as an "unstoppable force".
He added the government should be prepared to "rule in" ground forces to tackle the issue and that advisors should also be sent to assist the Iraqi army.
Speaking about IS, Lieutenant Lamb said: "Now we need to crush them.
"I'm not talking about large-scale conventional forces because Isil don't represent that form of threat.
"But I am talking about advisers, special forces, airborne brigades, for instance, the battalion of the landing of the US marine corps, those sort of capabilities which ourselves, the United Kingdom, and the United States have. But we should be prepared to rule them in."
Tony Blair has said the West should not rule out using ground troops to fight Islamist extremists in the future if it is "absolutely necessary".
The British government has already supplied arms to Kurdish fighters on the front line.
But the former Prime Minister said air strikes alone would not be enough to defeat IS.
In an essay, he wrote while training and equipping local fighters with weapons may work the option of sending in combat soldiers "should not be ruled out".
He said: "Provided that there is the consent of the population directly threatened and with the broadest achievable alliance ... we have, on occasions, to play our part."
Airpower alone will not defeat extremists like the Islamist State militants currently holding large swathes of territory in northern Iraq and parts of Syria, Tony Blair has said.
In an essay, the former Prime Minister wrote: "Air power is a major component of this, to be sure, especially with the new weapons available to us.
"But - and this is the hard truth - air power alone will not suffice. They can be hemmed in, harried and to a degree contained by air power. But they can't be defeated by it."
Mr Blair, whose premiership came to be defined by the Iraq War, acknowledged there was "no appetite" for ground engagement against IS.
But he warned: "You cannot uproot this extremism unless you go to where it originates from and fight it."
Western powers including the United Kingdom and the US should be prepared to commit ground forces in the fight against extremists like the Islamic State, former Prime Minister Tony Blair has said.
In an essay, Mr Blair argues that air strikes alone will not be enough to defeat IS or similar groups, and while training and equipping local fighters may work, the option of sending in combat soldiers should not be ruled out.
Mr Blair, whose premiership came to be defined by the Iraq War, acknowledged there was "no appetite" for ground engagement against IS but warned: "You cannot uproot this extremism unless you go to where it originates from and fight it."
He said the struggle against Islamist extremism should be seen as an international fight rather than a series of isolated conflicts, comparing it to the fascist and communist ideologies of the last century.
In an essay on his Tony Blair Faith Foundation's website the former premier also stressed the importance of engaging with a wider spectrum of radical Islamism, not just the violent fringe.
He said because extremists such as IS - formerly known as Isis - are "fanatical" and "prepared both to kill and to die" there could be no solution that does not involve force "with a willingness to take casualties in carrying the fight through to the end".
"This is where we get to the rub. We have to fight groups like Isis," he said.
"There can be an abundance of diplomacy, all necessary relief of humanitarian suffering, every conceivable statement of condemnation which we can muster, but unless they're accompanied by physical combat, we will mitigate the problem but not overcome it."
Former prime minister Tony Blair has welcomed the announcement of an open-ended ceasefire in Gaza.
The Middle East peace envoy congratulated the Egyptian government on its successful negotiations between Israel and the Palestinian factions.
The Quartet will now concentrate on a long-term plan for Gaza and for its reconstruction, including the effective and efficient reopening and reconnection to the outside world under the authority of the Palestinian authority government.
Such a plan will enable a proper and decent life for the people of Gaza, as well as protect the security of the people of Israel.
It is only tragic that such a ceasefire has come too late to save the lives of so many innocent people.
Former prime minister Tony Blair could be called before a parliamentary inquiry to explain a 'secret' deal he made with Sinn Fein during the Irish peace process, the Sunday Telegraph reported.
The deal saw 200 letters of assurances sent to fugitives known as 'on-the-runs' telling them they were not wanted by the police in the UK.
However, it did not rule out future prosecutions if new evidence emerged.
MPs on the Northern Ireland Affairs Committee are investigating the collapse of the case against John Downey, who was wrongly assured he was not wanted, when in fact Metropolitan Police were looking for him in connection with the 1982 Hyde Park bombings.
The Committee may now seek Mr Blair to give evidence as his role in orchestrating the deal is coming under increasing scrutiny.
Tony Blair should be impeached for misleading the Commons on the eve of the Iraq war, the UK's most senior MP has said.
Father of the House Sir Peter Tapsell said backbench MPs should use their "ancient" powers to haul the former prime minister before Parliament.
Sir Peter, who first entered the Commons in 1959, was one of a handful of Conservative MPs to vote against the Iraq war in 2003.
He challenged David Cameron to act during Prime Minister's Questions today, but Mr Cameron failed to directly address the query.
Boris Johnson has launched a pointed personal attack on former prime minister Tony Blair over his claims that the current conflict in Iraq is not linked to the West's 2003 invasion.
In his Daily Telegraph column, Mr Johnson said Mr Blair had "finally gone mad" and his "unhinged" attempt to rewrite history is undermining arguments for Western intervention in Iraq.
"In discussing the disaster of modern Iraq he made assertions that are so jaw-droppingly and breathtakingly at variance with reality that he surely needs professional psychiatric help," the London Mayor added.
He said that by refusing to accept that the 2003 war was "a tragic mistake", "Blair is now undermining the very cause he advocates: the possibility of serious and effective intervention.
"Somebody needs to get on to Tony Blair and tell him to put a sock in it, or at least to accept the reality of the disaster he helped to engender. Then he might be worth hearing," Mr Johnson concluded.
A former British Ambassador to the United States has supported former Prime Minister Tony Blair's calls for the international community to look at a renewed military intervention in Iraq.
Sir Christopher Meyer told ITV News Britain does not want to "get caught in someone else's civil war", but said Iraq as an ally should be given "appropriate help".
ITV News Political Correspondent Romily Weeks reports: