Mr Obama said the battle against IS, also known as Isis, was not “America’s fight alone” as he revealed details of a plot by another terrorist group to attack targets in Europe and the United States.
At least 70 IS fighters and 50 other militants, said to have al-Qaeda links, were reportedly killed in the air strikes on Monday night. US officials said the attack on the Khorasan group, an al-Qaeda affiliate, followed intelligence it was at an advanced stage of a plot possibly involving improvised explosive devices using hand-held electronic devices or toiletries which could be taken onboard an aircraft.
Mr Obama said Saudi Arabia, the United Arab Emirates, Jordan, Bahrain and Qatar had played a role in the attacks on IS, the group responsible for the murders of US journalists James Foley and Stephen Sotloff, and British aid worker David Haines.
Last night Barbara Henning, the wife of British hostage Alan Henning, from Salford, made a renewed plea for his release after receiving an audio file of him pleading for his life.
She said: “I have seen Muslims across the globe question Islamic State over Alan’s fate.
“He was working with Muslims to help the most vulnerable within Syria. He went to Syria to help his Muslim friends deliver much-needed aid.”
She continued: “We are at a loss why Islamic State continue to threaten his life.
“I have been told that he has been to a Sharia court and found innocent. I implore Islamic State. Please release Alan.”
While the UK has not joined in the military action, Prime Minister David Cameron has voiced his support.
He will be the first Prime Minister to hold face-to-face bilateral talks with an Iranian president since the Islamic revolution of 1979 when he meets Hassan Rouhani today.
Yesterday, Mr Rouhani said the military action in Syria had “no legal standing”, but stopped short of condemning it.
Speaking at the White House, Mr Obama said the US would “do what’s necessary” to defeat IS.
He said the backing of five Arab states underlined the support he was building for international action to defeat IS – also referred to as Isil [Islamic State of Iraq and the Levant].
“The strength of this coalition makes it clear to the world that this is not America’s fight alone,” he said. “The overall effort will take time, there will be challenges ahead, but we are going to do what’s necessary to take the fight to this terrorist group for the security of the country and the region and for the entire world.”
Later United Nations secretary-general Ban Ki-Moon welcomed air strikes against militants in Syria.
He cautioned, however, that parties involved in the air strikes must abide by international law and minimise civilian casualties.
Earlier, Mr Cameron – in New York for the UN general assembly – voiced his backing for air strikes. He will use the two-day gathering to rally support for international action.
Downing Street said: “[He] supports the latest air strikes … [he] will be holding talks at the UN over the next two days on what more the UK and others can do to contribute to international efforts to tackle the threat we all face from Isil.
“The UK is already supplying arms to the Kurds as well as surveillance operations by a squadron of Tornadoes and other RAF aircraft.”
Mr Cameron is expected to use a speech today to set out further details of Britain’s contribution. There is growing speculation he will announce that UK forces will join the air strikes – although only in Iraq, at the invitation of the Baghdad government.
The Ministry of Defence said no decisions had yet been taken on any UK action against the militants who have seized swathes of Iraq and Syria, carrying out atrocities.
While the US – like Britain – has ruled out any co-operation with the regime of Syrian president Bashar al-Assad, Damascus said Washington had informed its UN envoy before the attacks began.
Among the areas hit was the IS stronghold of Raqqa where it is thought Mr Henning, is being held. Since August, the US has carried out around 190 air strikes against IS in Iraq as part of Mr Obama’s campaign to “degrade and destroy” the militants.
Britain has been helping supply arms to Kurdish peshmerga forces fighting IS in northern Iraq, and offering support to the new Iraqi government led by Haider Abadi.
There are concerns over the legality of the air strikes in Syria without the explicit backing of the Damascus regime, unlike the military action in Iraq.
Iran’s president, Mr Rouhani said: “The bombardment must have a certain framework that is needed to take place in a third country.”
He said without a UN mandate or a request from the affected country, military interventions did not have “any legal standing”.
Scottish external affairs minister Humza Yousaf said: “Isil is a global threat that must be tackled.
“The Scottish Government has consistently said that any military action should conform to international law, be sanctioned by the UN and receive parliamentary consent.”