U.S. President Barack Obama is warning of a "firm response" should North Korea go ahead with a fourth nuclear test that some analysts and officials say could take place in the coming days.
Speaking to South Korean media before landing in Seoul, Mr. Obama said "Pyongyang will gain absolutely nothing from another nuclear test except to deepen its own isolation from the international community."
South Korea's government has warned that the North appears to be making preparations for another test at its main nuclear test site, an assertion backed up by a U.S.-based research group.
The U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies says the most recent satellite images of the Punggye-ri nuclear test site show activity "probably related to preparations for a detonation."
It said the images, taken Wednesday, showed "probable command and control vehicles" parked in the main support area of the test site, noting similar vehicles were spotted in the area prior to the North's February 2013 test.
But the institute warned it is not clear if Pyongyang, whose leaders are notoriously unpredictable, will follow the same pattern this time.
Leonid Petrov, a veteran Korea analyst at the Australian National University, sees another North Korean nuclear test as inevitable.
"I don't believe the fourth nuclear test is going to happen during President Obama's visit in South Korea, but it will come sooner or later, and I believe it may happen this year, particularly given the satellite images."
There has been widespread speculation about what type of nuclear device North Korea will detonate this time around. Its first three tests - in 2006, 2009, and 2013 - are believed to have used plutonium.
Petrov says the North's next test could employ uranium, which is easier and cheaper to acquire. Or he says a combination of plutonium and uranium is possible.
"And that is going to become something sensational, simply because the thermonuclear technology is much more powerful than the traditional technology in nuclear devices that have been tested by North Korea before. If that happens, it means that they need significantly less fissile material such as plutonium and uranium, but the yield of the detonation is going to be a hundred times more."
South Korean Foreign Minister Yun Byung-se acknowledged this week that a fourth nuclear test would be a "game changer," although he did not elaborate.
Besides the technological advances achieved by a possible North Korean nuclear test, the move might also be a "game changer" in terms of diplomacy.
President Park Geun-hye, who took office in 2013 on promises of improving ties with Pyongyang, has yet to have to respond to a North Korean nuclear test.
If a detonation does take place, Petrov warns that President Park may feel pressured to change her stance to at least temporarily become less flexible on her stance toward North Korea.