President Obama and British Prime Minister David Cameron will use this week's NATO summit to press for a multi-national coalition to carry out military action against the Islamic State militant group, formerly known as ISIS.

The summit began Thursday in Wales with a session on Afghanistan, with the Islamic State threat looming over the discussion.

The two leaders, ahead of the session, began laying the groundwork for their Middle East mission in a joint op-ed published in Thursday's edition of The Times of London. In the piece, Obama and Cameron vowed that their leaders would "not be cowed" by the extremists who have beheaded two American journalists and threatened a British aid worker with the same fate in recent weeks.

"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 op-ed states.

Meanwhile, the Associated Press reported that over the past day, Secretary of State John Kerry and other administration officials have reached out to leaders from Australia, the United Arab Emirates, Jordan, Qatar, Saudi Arabia, Italy and Israel to discuss how to combat ISIS. State Department spokeswoman Jen Psaki said the discussions focused on what each country might contribute -- including weapons, humanitarian aid and other resources -- and noted that some nations already have.

Obama "wants to build an international coalition," Psaki said. "That's not going to be overnight. We need capabilities from many countries."

Obama has been under pressure from members of Congress to broaden the U.S. offensive against ISIS. That criticism increased last week when Obama admitted that "we don't have a strategy yet" to address the militant group in Syria, and deepened when the president first said that his goal was to "degrade and destroy" ISIS Wednesday, before changing tone and saying that he thought the U.S. could make ISIS a "manageable problem" if American forces were part of an international coalition.

"Are we going to contain ISIS or are we going to crush ISIS? And the president has not answered that," Rep. Adam Kinzinger, R-Ill., told Fox News, reacting to the president's remarks.

Administration officials said Wednesday that that the U.S. will not launch a ground war against the Islamic State militants. But they stopped short of ruling out airstrikes against the group in its safe haven in Syria, as the U.S. has resisted for years.

Obama has "been clear that we're not going to be limited by geography," State Department spokeswoman Jen Psaki said. She described a range of actions being considered against the Islamic State, and noted that decisions and discussions were ongoing.

With the summit beginning in Wales Thursday, the Daily Mail reported that Cameron's government was not only leaning toward joining U.S.-led strikes against ISIS in its northern Iraq stronghold, but was also ready to consider sending British forces to Iraq as part of a possible NATO force to train Iraqi and Kurdish soldiers. The Daily Mail reported added that such an action would be contingent on a request by Baghdad.

"We need to show real resolve and determination, we need to use every power and everything in our armory with our allies -- with those on the ground -- to make sure we do everything we can to squeeze this dreadful organization out of existence," Cameron told the British network ITV Thursday.

In contrast to last year, when Cameron's proposal to take part in planned airstrikes against Syria over President Bashar al-Assad's use of chemical weapons suffered was defeated in the House of Commons, the Daily Mail reported that all three major British political parties are either at or on the brink of supporting U.K. involvement in airstrikes.

The Associated Press contributed to this report.

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