Nicholas Kamm, Associated Press
US Secretary of State John Kerry speaks at a press conference following talks with King Abdullah II of Jordan and Israeli Prime Minister Benjamin Netanyahu in Amman on November 13, 2014 to discuss the upsurge in violence in east Jerusalem and the West Bank and other regional issues. Kerry issued a warning Monday to Islamic State militants that "we are not intimidated" after another American hostage was killed.

WASHINGTON — Secretary of State John Kerry issued a warning Monday to Islamic State militants that "we are not intimidated" after another American hostage was killed.

Kerry said the brutality of the Islamic State group and its potential spread worldwide as a key reason, among many, that the United States must remain deeply engaged in the Mideast.

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.

But the bulk of his comments sought to underline the case for deep U.S. involvement in the Mideast. "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," Kerry said.

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."

A day earlier, the White House confirmed the death of U.S. 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. The militant group that controls much of northern Syria and Iraq has now killed five Westerners it was holding.

The IS video of a militant boasting about killing Kassig also appears to show extremists beheading a dozen Syrian soldiers. Kassig was captured by the extremists in eastern Syria on Oct. 1, 2013, while delivering relief supplies for the aid group he founded.

Left unchecked, Kerry said that the Islamic State group could grow worldwide. Already, he said, the IS has seized more land and resources "than al-Qaida ever had on its best day of its existence."

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

Immediately after the speech, Kerry headed to London, where he will hold talks with European and Mideast officials on the Iran negotiations as well as volatile situations in the Mideast.

From London, Kerry will travel to Vienna, where the next round of nuclear talks is set to begin on Tuesday and continue through the week, the State Department says.

Kerry may not take part in all the negotiating sessions in Vienna, and other stops are possible, officials said Monday.