The United States is “not intimidated” by the Islamic State (ISIS) terrorist group, Secretary of State John Kerry said Monday, after another American hostage was killed by the group.

Kerry said the brutality of the Islamic State group and its potential spread worldwide was a key reason, among many, that the United States must remain deeply engaged in the Middle East, according to the Associated Press (AP).

His comments came right before he headed overseas for nuclear talks with Iran as a November 24 deadline for a deal looms.

“We are obviously entering in a key period with negotiations regarding Iran’s nuclear program,” Kerry said at an annual policy forum in Washington, hosted by the publisher of Foreign Policy magazine.

“We have to be deeply engaged — deeply engaged — in this region, because it is directly in the interest of our national security and our economy, and it is also in keeping of who we are,” he added.

“The United States does not go in search of enemies in the Middle East. There are times, however, and this is one, when enemies come in search of us,” said the Secretary of State.

The comments came one day after the White House confirmed the death of aid worker Peter Kassig, a former soldier who tried to help wounded Syrians caught in their country’s civil war but ended up dying himself at the hands of Islamic State.

Left unchecked, Kerry said that the Islamic State group could grow worldwide. Already, he said, ISIS has seized more land and resources “than Al-Qaeda ever had on its best day of its existence.”

ISIS “leaders assume that the world will be too intimidated to oppose them,” Kerry said. “But let us be clear: We are not intimidated.”