'Preparing options'
On Monday evening, US officials said Mr Obama had approved over the weekend reconnaissance flights by unmanned and manned aircraft.
An F/A-18C Hornet returning from Iraq lands on USS George H W Bush (15 August 2014)
Fighter jets based on the aircraft carrier, USS George H W Bush, have been carrying out air strikes in Iraq
One official later told the Associated Press that they had already begun.
The US military has been carrying out aerial surveillance of IS - an al-Qaeda breakaway formerly known as Isis - in Iraq for months and launched air strikes on 8 August.
The president cited the threat to US diplomats and military personnel and the humanitarian crisis in the north, where hundreds of thousands of people have fled their homes since June as IS fighters and allied Sunni rebels have taken control of dozens of cities, towns and villages.
Mr Obama has resisted taking military action in Syria, but Pentagon officials are said to have advised him that the only way the threat from IS can be fully eliminated is to go after the group there.
A spokesman for Gen Martin Dempsey, chairman of the US Joint Chiefs of Staff, said the Pentagon was "preparing options to address Isis both in Iraq and Syria with a variety of military tools including air strikes".