North Korea has apparently sealed the tunnel to its nuclear test site, a significant sign that it may carry out its fourth nuclear test imminently, according to an official in Seoul.
The move came as US President Barack Obama was due to arrive in Seoul today and follows warnings from Pyongyang over military drills carried out by US and South Korean forces.
A South Korean official said North Korea had placed fissile materials and related equipment in a tunnel at its underground nuclear test site at Punggye-ri, and sealed the tunnel entrance.
The sealing is crucial. According to the official, any country that seals nuclear material in an underground facility must carry out a detonation "within seven to 14 days" or unseal the tunnel and remove the materials.
"This is a technical issue," explained Shim Chang-hoon of Seoul think tank the Asan Institute. "If they want to conduct a nuclear-weapons test and have put the facilities inside, they cannot keep them there for longer than 15 days."
The official declined to say when the tunnel was sealed, but South Korea's Defence Ministry had announced activity at Punggye-ri on Tuesday.
If that activity included the tunnel sealing, a test will take place by May 6. South Korean officials had previously said a detonation awaited only a "political decision" from Pyongyang.
But not everyone agrees with Seoul's analysis. Influential website 38 North, citing satellite images, said it did not anticipate a test, given the apparent lack of communications gear at the site.
And South Korean officials have also said that the activity could be a bluff.
Punggye-ri, in the country's desolate northeast, is the site of 2006, 2009 and 2012 nuclear tests.
Today, Obama arrives in Seoul for the Korean leg of his Asian tour, of which Pyongyang media have been critical. South Korean and the US will also finish joint air-force exercises today.
"If you really want to send a message and express displeasure, what better time?" asked Dan Pinkstone of the International Crisis Group.
"It would be consistent with the regime's past behaviour."
Pyongyang has tested missiles and nuclear devices on sensitive or symbolic dates.
In 2006, North Korea detonated its first nuclear device while Shinzo Abe, during his first stint as Japan's prime minister, was visiting then South Korean president Roh Moo-hyun in Seoul. In 2009, it launched missiles on the US national holiday, July 4. And it test-fired two medium-range missiles on March 25 - the day that Obama hosted a summit with Abe and South Korean President Park Geun-hye.
"A nuclear test is an important card - like a joker - in power games," said Shim. "If North Korea believes that it has a bullying effect on the outside world, the dates of the visit would be one opportunity."