It comes after a shallow quake measuring 6.3 on the Richter scale was detected near the rogue nation's only known nuclear testing site at Punggyeri, in the North Hamgyong Province.
South Korea immediately convened a national security council meeting after the tremor, which US seismologists say was recorded at a depth of six miles (10 km).
North Korea in July conducted its first ever ICBM tests, part of a stunning jump in progress for the country's nuclear and missile program since Kim rose to power.
North Korea's nuclear test Sunday was apparently its most powerful yet.
Yukiya Amano added: "This new test, which follows the two tests past year and is the sixth since 2006, is in complete disregard of the repeated demands of the global community".
"Every country with five nuclear explosions was well on its way to thermonuclear weapons by the fifth test", Lewis added.
South Korean President Moon Jae-in also called for the "strongest punishment" against Pyongyang, including new United Nations sanctions "to completely isolate North Korea". Talking is not the answer!"Trump said in a Twitter post". The Korea Meteorological Administration estimated that the nuclear blast yield Sunday was between 50 to 60 kilotons.
Another threat is deemed to be looming ahead as North Korean leader Kim Jong Un has been spotted inspecting a new, ultra explosive hydrogen bomb.
China, North Korea's only major ally, condemned the test.
South Korea and Japan said the North may have conducted a sixth nuclear test, according to reports from the Blue House and Kyodo.
The U.S. Geological Survey called the first quake an explosion with a magnitude 6.3.
An undated photo of the North Korean leader "providing guidance" with the hydrogen bomb accompanied the claim.
The United States and South Korea responded with a mock strike drill over the Korean peninsula.
Seoul's presidential office doubts this could be the North's sixth nuclear test.
Kim's regime has produced a miniaturized nuclear warhead that can fit atop its missiles, surpassing a key technological hurdle, the Washington Post reported August 8, citing an Defense Intelligence Agency analysis completed in July.
USA shoots down ballistic missile in test amid recent North Korean provocations
The previous test in June involved the John Paul Jones firing SM-3 missiles at a ballistic target, and was not successful. Earlier this week, an intermediate-range missile, identified by the North Koreans as a Hwasong-12, was sacked over Japan.