North Korea said Thursday that its latest missile tests involved a new hypersonic system aimed at strengthening its nuclear war deterrent, as leader Kim Jong Un continues to build up weapons designed to overwhelm South Korea defenses.

Tokyo and nearby Seoul both have expressed concern over North Korea's development of long-range, nuclear-capable ballistic missiles in defiance of multiple U.N. prohibitions. North Korea conducts frequent missile tests that send projectiles over Japan or into nearby waters.

North Korea has made significant advances in its nuclear and ballistic missile programs in the absence of talks.

Donald Trump describes North Korea as "sort of a nuclear power" ahead of his Asia trip and potential meeting with Kim Jong Un.

Experts had said North Korea could conduct provocative missile tests before or during a regional summit in South Korea as it seeks to gain recognition as a nuclear weapons state.

South Korean Unification Minister Chung Dong-young told a press conference that North Korea is presumed to possess up to 2,000 kilograms of highly enriched uranium and it is trying to dramatically expand its nuclear arsenal, local Yonhap news agency reported.

Kim Jong Un says North Korea will only engage in talks if the U.S. drops denuclearization demands, creating a diplomatic stalemate with the Trump administration.

The foreign ministers of North Korea and China have agreed to deepen bilateral ties and resist hegemonism, a likely reference to pushing back against the United States

A North Korean soldier defected to South Korea across the heavily fortified Demilitarized Zone​ on Sunday, South Korea's military said.

At a military parade earlier this month, Kim Jong-un unveiled some of his military's newest weapons, including what appeared to be a short-range ballistic system fitted with hypersonic glide vehicles.