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North Korea conducted its second nuclear test in eight months on Friday, raising concerns that Pyongyang has moved a step closer to its goal of a nuclear-armed missile that could one day strike the U.S. mainland.

State TV said the atomic detonation — the fifth carried out by Kim Jong Un's isolated regime — "put on a higher level [the North's] technology of mounting nuclear warheads on ballistic rockets."

"It would be smaller than both Hiroshima or Nagasaki, but it would still rip the heart out of a city"

Seoul estimated the test produced the North's biggest-ever estimated explosive yield.

The announcement came hours after South Korea noted unusual seismic activity near its neighbor's northeastern nuclear test site.

The U.S. Geological Survey detected a seismic event that it called an "explosion" at around 9:30 a.m. Friday Seoul time (8:30 p.m. Thursday ET) about 12 miles from Sungjibaegam, North Korea.

A South Korean military official later said it "was in fact North Korea’s fifth nuclear test," adding that it appeared to have the "impact" of a 10-kiloton explosion.

What does that mean exactly?

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"It would be smaller than both Hiroshima or Nagasaki, but it would still rip the heart out of a city," IHS Jane's analyst Karl Dewey explained. "If detonated in the middle of Lower Manhattan in New York, for example, it would destroy pretty much everything up to the West Village and down to about the New York Stock Exchange."

Jeffrey Lewis of the California-based Middlebury Institute of International Studies told Reuters the highest estimates of seismic magnitude suggested this was the most powerful nuclear test conducted by North Korea so far.

"The important thing is that five tests in, they now have a lot of nuclear test experience. They aren't a backwards state any more," Lewis added.

The 5.0-magnitude earthquake Friday is the largest of the four past quakes associated with North Korean nuclear tests, according to South Korea's weather agency. Artificial seismic waves measuring 3.9 were reported after North Korea's first nuclear test in 2006; 4.8 was reported from its fourth test this January.

State TV said the test "examined and confirmed" specific features of a nuclear warhead designed to be mounted on a medium-range ballistic missile, which it most recently tested on Monday when President Barack Obama and other world leaders were gathered in China for the G-20 summit.

Obama called Friday's test a "grave threat to regional security" and said he'd spoken to the South Korean and Japanese leaders, who agreed to work with the U.N. Security Council on "additional significant steps, including new sanctions" for North Korea.

"To be clear, the United States does not, and never will, accept North Korea as a nuclear state," Obama said in a statement. "Today's nuclear test, a flagrant violation of multiple U.N. Security Council Resolutions, makes clear North Korea's disregard for international norms and standards for behavior."

South Korean President Park Geun-hye accused Kim's regime of "fanatical recklessness," according to a spokesperson.

A statement released by Park's office said the nuclear test "is different from the past in terms of the intensity."

North Korea’s nuclear test, if confirmed, is a deeply troubling & regrettable act, says IAEA Director General #DPRK pic.twitter.com/T6DkFAypEi — IAEA (@iaeaorg) September 9, 2016

After the North's fourth test in January, South Korean lawmaker Lee Cheol Woo said Seoul's National Intelligence Service told him that an estimated explosive yield of six kilotons was detected.

North Korea's latest nuclear test coincided with the anniversary of its 1948 foundation as a republic. Pyongyang said no radioactive material leaked, but the explosion put the region on edge.

China, North Korea's only major diplomatic ally, said it was resolutely opposed to the test and urged Pyongyang to stop taking any actions that would worsen the situation.

Japan's Chief Cabinet Secretary Yoshihide Suga called North Korea an "outlaw nation in the neighborhood."

Secretary of State John Kerry said he spoke with the foreign ministers of Japan and South Korea, adding that "everybody shares concerns" about the situation on the Korean peninsula.

North Korea is believed to have conducted at least four nuclear tests since 2006, including the one in January that the country claimed was a miniaturized hydrogen bomb. Experts have cast doubt on that claim.

Pyongyang also has conducted several recent missile tests, including a submarine-launched ballistic missile in August. Obama has vowed tougher sanctions in response.

In July, North Korea said it had conducted a ballistic missile test that simulated pre-emptive strikes against South Korean ports and airfields used by the U.S. military.

The U.S. and South Korea in July announced they decided to deploy an advanced missile system, the Terminal High Altitude Area Defense, to South Korea.

North Korea and South Korea remain technically at war, as a conflict from 1950 to 1953 ended in an armistice.