WASHINGTON, DC – Defense Secretary James Mattis called North Korea the “most urgent and dangerous threat,” effectively demoting Russia from the top position.

“The most urgent and dangerous threat to peace and security is North Korea. North Korea’s continued pursuit of nuclear weapons and the means to deliver them has increased in pace and scope,” he said in prepared testimony to the House Armed Services Committee on Monday evening.

“The regime’s nuclear weapons program is a clear and present danger to all, and the regime’s provocative actions, manifestly illegal under international law, have not abated despite United Nations’ censure and sanctions,” he added.

At his confirmation hearing with the Senate Armed Services Committee in January, he said Russia was the “principal threat” to the U.S.’s security, in line with former Defense Secretary Ashton Carter. However, that was before North Korea began speeding up its development of its ballistic missile program this year.

North Korea has test-fired ballistic missiles in defiance of international law on nine occasions this year, a dramatic uptick of five test-launches last year.

Chairman of the Joint Chiefs of Staff Marine Gen. Joseph Dunford stuck by his previous ranking of Russia as the greatest threat, listing the five key challenges as Russia, China, Iran, North Korea, and violent extremist organizations.

Both men talked about how catastrophic war with North Korea would be.

Upon questioning by Rep. Martha McSally (R-AZ), Mattis said war with North Korea would be a “war like nothing we’ve seen since 1953.”

“It would be a very, very serious war,” he added.

Dunford pointed out there are 25 million people in Seoul, South Korea, including 300,000 Americans, who would be at risk.

He said they would be “within range of thousands of rockets, missiles and artillery pieces along the border.”

“In that conflict, we will see casualties, unlike anything we’ve seen in 60 or 70 years, and many of those casualties will be in the first three, five, seven days of the war where all those people in the Greater Seoul area will be exposed to that North Korea threat that we will not be able to mitigate initially,” he added.

But, he said, “No doubt in my mind, if we go to war with North Korea, we will win the war. We will be successful in accomplishing our objectives.”

McSally, a retired Air Force A-10 pilot and squadron commander, agreed.

“I just think American people need to understand the gravity of the situation that we’re dealing with… this is something like we’ve never seen in my lifetime should deterrence fail,” she said.