South Korea warns North Korea is readying more tests after threatening to nuke the Pacific

Thomson Reuters

A file photograph of Kim Jong Un inspecting a Hwasong-12 (Mars-12) ballistic rocket.

• South Korea's spy agency says North Korea looks like it's getting ready for a missile or nuclear test.
• North Korea has threatened to detonate a nuclear missile over the Pacific, which could poison the ocean or lead to a strong US response.
• North Korea's usual test site for nuclear devices seems damaged, but still accessible.

South Korea's intelligence service warned on Thursday that it saw "active movement" at North Korea's missile research facility and that the world should expect more nuclear and missile testing from Pyongyang, according to Yonhap News.

"The North will carry out additional nuclear tests and continue to push for the development of miniaturized, diversified nuclear warheads," South Korea's National Intelligence Service said, according to lawmakers who spoke to Yonhap.

Additionally, the spy agency noted that North Korea has had some trouble with its nuclear test site, that a recent thermonuclear blast rocked so hard that landslides and cave-ins have apparently restricted access to the site.

On Tuesday, Japan's Asahi TV quoted a North Korean source as saying that cave-ins in tunnels at the nuclear test site caused the death of up to 200 workers.

South Korea's NIS, however, maintained that North Korea still had access to the site via an alternate tunnel, and North Korea denied the death of the workers.

North Korea has recently floated the idea of conducting above-ground, or atmospheric testing instead.

A CNN reporter quoted a North Korean official as saying the US should take "literally" its threat to detonate the "strongest hydrogen bomb test over the Pacific Ocean."

Separately, North Korea has threatened to launch a salvo of missiles at the US territory of Guam.

To do so, North Korea would have to fire a medium to long-range missile. North Korea hasn't tested a missile in over a month, it's longest pause since it began launching missiles in February.

The US maintains a large naval presence in the Pacific with advanced ballistic missile defense capabilities.

Experts have told Business Insider that if the US assesses North Korea may fire a ballistic missile with a nuclear warhead, the military may look to shoot it down.