I.S. has been persecuting tens of thousands of Christians and other religious minorities in Iraq and Syria.
The extremist group has been taking over large parts of the countries.
The Prime minister and Barack Obama wrote in the Times they were determined to "confront" I.S. and that some countries misunderstood the situation.
A joint letter in the paper was published as the two leaders prepare to meet for the NATO summit in Wales.
They said Britain and America would "not waver in our determination to confront" the group and that "countries like Britain and America will not be cowed by barbaric killers."
"Developments in other parts of the world, particularly in Iraq and Syria, threaten our security at home," they said.
Tens of thousands of Christians have been forced to flee their homes by the terrorist group with many being killed by the extremist Islamic fighters.
The Foreign Secretary said air strikes had not been ruled out but at the moment the UK would continue with its humanitarian mission only.
There have also been more calls for the UK and US to hold talks with Syrian President Assad in the struggle to tackle the extremists.
Both countries have ruled that out.