HONG KONG — The United States said on Tuesday that it had begun deploying an advanced and contentious missile defense system in South Korea, prompting China to warn of a new atomic arms race in a region increasingly on edge over North Korea’s drive to build a nuclear arsenal.

The American announcement came a day after the simultaneous launch of four missiles by North Korea into waters off the Japanese coast, which Pyongyang said was a drill for striking American bases in Japan. The feat, footage of which was broadcast on state television, raised concern about the North’s ability to overwhelm the new defense system being deployed.

Hours later, North Korea further unnerved the region by declaring it was blocking all Malaysians from leaving its soil, sharply escalating a dispute over last month’s assassination of Kim Jong-nam, the half brother of North Korea’s dictator, Kim Jong-un.

Malaysia has accused several North Korean citizens of using VX nerve agent to kill Mr. Kim in a case that has reminded the world of Pyongyang’s access to a stockpile of banned chemical weapons on top of its nuclear program — and its willingness to take extreme measures.

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Featured image courtesy of EPA.