North Korean leader Kim Jong Un sent a fresh warning to the U.S., saying that he would boost the country’s nuclear capabilities — just a day after Pyongyang claimed it had successfully tested a new “hypersonic missile” that could evade U.S. missile defenses.

“As the actions of the past eight months of the new U.S. administration have clearly shown, the U.S. military threat and hostile policy toward North Korea has not changed at all,” Kim said in a speech on the second day of a parliament meeting, the official Korea Central News Agency reported Thursday.

Washington’s North Korea policy is becoming “more cunning,” he added, saying that he would step up new weapon developments to deter any possible “military provocations by hostile forces.”

Foreign media reported that the United States appealed for diplomacy after decades of strained relations.

“The United States harbors no hostile intent toward the DPRK. Our policy calls for a calibrated, practical approach that is open to and will explore diplomacy with the DPRK to make tangible progress that increases the security of the United States, our allies, and our deployed forces,” a State Department spokesperson told Yonhap News Agency Wednesday in an email. “We are prepared to meet with the DPRK without preconditions. We hope the DPRK will respond positively to our outreach.”

Kim’s speech came just a day after Pyongyang declared that it had successfully tested of a new “hypersonic missile,” putting the Kim regime closer to putting nuclear warheads on what it called “high-speed gliders that can evade U.S. missile defenses.”

At the same time, Kim extended an olive branch to South Korea, saying he was willing to restore inter-Korean communications lines and that Seoul must forfeit its “hostile policy” before the two countries can declare an official end to the Korean War. The countries are still technically at war after the 1950-53 conflict ended in an armistice rather than a peace treaty.

South Korean President Moon Jae-in reiterated his call for formally ending the Korean War in an address to the United Nations General Assembly last week. Moon has less than a year left in office and is looking to make good on a key policy priority of reconciliation with North Korea.

Kim Yo Jong, the influential sister of North Korean leader Kim Jong Un, reached out to South Korea Friday and said Moon’s proposal to officially declare an end to the Korean War is an “interesting and good idea” in that it suggests a cessation of hostilities between the two sides.