CARDIFF, Wales — President Obama and Prime Minister David Cameron of Britain have called on NATO to reject “isolationist” impulses and confront the rising terrorist threat posed by Sunni militants in the Middle East, saying the United States and Britain “will not be cowed by barbaric killers.”

“We will not waver in our determination to confront” the militant group known as the Islamic State in Iraq and Syria, or ISIS, the two leaders wrote in a joint opinion piece published in Thursday’s editions of The Times of London. “If terrorists think we will weaken in the face of their threats they could not be more wrong.”

Their pointed words came as leaders gathered in Wales for a NATO summit meeting that was intended to focus on responding to Russia’s escalating military intervention in Ukraine. On the sidelines of the meeting, however, American and British efforts to assemble and lead an international coalition against ISIS are expected to predominate.

“The international community as a whole has an obligation to stop the Islamic State from advancing further,” Anders Fogh Rasmussen, the NATO secretary general, said at a news conference on Thursday. But he noted that there had been no request from Iraq for NATO assistance in confronting the group.