US President Barack Obama and British Prime Minister David Cameron declared Thursday that their nations will "not be cowed" by extremists who have killed two American journalists

US President Barack Obama and British Prime Minister David Cameron declared Thursday that their nations will "not be cowed" by extremists who have killed two American journalists.

"We will be more forthright in the defense of our values, not least because a world of greater freedom is a fundamental part of how we keep our people safe," the leaders wrote in a joint editorial in the Times of London.

Their comments come as world leaders gather at a golf resort in Wales for a high-stakes NATO summit. While the official agenda will focus on the crisis in Ukraine and the drawdown of the NATO combat mission in Afghanistan, the rise of the so-called Islamic State group in Iraq and Syria will dominate discussions on the sidelines of the summit.

The militants have claimed responsibility for murdering two American journalists, releasing gruesome videos of their beheadings. Both the U.S. and Britain are deeply concerned about the potential threat to their homelands that could come from the foreign fighters who have joined the violent Islamic State group.

Cameron on Monday proposed new laws that would give police the power to seize the passports of Britons suspected of having traveled abroad to fight with terrorist groups.

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.