North Korea's young dictator is said to be "extremely nervous" about being assassinated.

Secretary of Defense Minister James Mattis. Samuel Corum | Anadolu Agency | Getty Images

A North Korean war would be "catastrophic," but the U.S. would ultimately prevail, U.S. Defense Secretary James Mattis told Congress on Thursday. "I will suggest that we will win," the retired four-star general said in testimony before the House Appropriations Subcommittee on Defense. "It will be a war more serious in terms of human suffering than anything we have seen since 1953." He added: "It will involve the massive shelling of an allies' capital [Seoul], which is one of the most densely packed cities on Earth. It would be a war that fundamentally we don't want. Our allies and us would win at great cost." Mattis also said Japan would be among the countries that would be potentially affected by such a "catastrophic war" with North Korea. "We're exhausting all possible diplomatic efforts," he said. Mattis said those efforts include continuing to press for help from China, a longtime ally of North Korea and its largest trading partner.

"North Korea is today a strategic burden to them; it's not a strategic asset," he said. "China has actually responded in some ways positive." Mattis was referring to China participating in a recent unanimous vote in the United Nations' Security Council for increased sanctions against Pyongyang. The council agreed to slap a travel ban and asset freeze on just over a dozen North Korean individuals and several entities, including the North's Koryo Bank, linked to the finances of key officials of the secretive state. Even so, some observers believe China could do more to pressure Pyongyang to abandon its nuclear and ballistic missile programs. "North Korea employs a global array of overseas networks to circumvent international sanctions and continue its pursuit of nuclear weapons," according to a "Risky Business" report released this week by the Washington think tank C4ADS. It added that China trade was "the largest market exploited by" the Pyongyang regime. In Seoul, meanwhile, South Korea's President Moon Jae-in on Thursday opened the door to dialogue with North Korea but laid out conditions that include stopping nuclear and ballistic missile tests. "I make it clear that if North Korea stops making additional nuclear and missile provocations, we can come to dialogue with North Korea without conditions," he said in remarks at a peace center in Seoul. The South Korean president said he's open to holding parallel talks on both denuclearization and a peace treaty to formally end the Korean War. "I am willing to discuss, sitting knee to knee and putting our heads together, how we will implement the existing inter-Korean agreements. We will be able to talk comprehensively about the complete dismantling of North Korea's nuclear program, the establishment of a peace regime on the peninsula, and even the normalization of North Korea-US relations."

People watch a television broadcast reporting the North Korean missile launch at the Seoul Railway Station on June 8, 2017 in Seoul, South Korea. Getty Images