President Barack Obama and British Prime Minister David Cameron say they will "not be cowed" by the Islamic militants who have killed two American journalists.

The leaders have written a joint editorial published in the Times of London ahead of Thursday's start to the NATO summit. Leaders are gathering at a golf resort in Wales for two days of talks on the crisis in Ukraine and the drawdown of combat missions in Afghanistan.

But the rise of the Islamic State group in Iraq and Syria will dominate discussions on the sidelines of the summit.

Obama and Cameron appear to suggest that NATO should play a role in containing the militants, but were not specific in what action they would seek from the alliance.