Analysts say recent construction work at a North Korean nuclear site suggests Pyongyang may be preparing to conduct a fourth underground nuclear test.
The U.S.-Korea Institute at Johns Hopkins University says new satellite images show two new tunnel entrances and other construction work at the North's Punggye-ri nuclear test site.
The report on the institute's blog, 38 North, said there are no signs a nuclear test is imminent. It said the tunnels could either be used for nuclear detonations or for other purposes such as ventilation.
But it warned that the ongoing construction indicates the North "is preparing to conduct additional detonations in the future as part of its nuclear weapons development program."
North Korea conducted underground nuclear tests in 2006, 2009, and in February of this year, in defiance of United Nations sanctions.
Much of the West's knowledge of North Korea's secretive nuclear program is obtained by examining satellite photos and other intelligence, which often provide only an incomplete picture.
Cedric Leighton, a security consultant and former U.S. Air Force intelligence officer, tells VOA the new construction at Punggye-ri could simply be repair efforts due to July floods, for instance.
"But it doesn't explain why there's so many new structures and new buildings going up. And that has made a significant difference in the structure of the site and could mean the site has received a new mission or is going to be set up in a way so that new nuclear tests could potentially take place."
Leighton agrees that no nuclear test seems imminent, but warns that there could be a new test "in several months' time or a year's time."
There have been other recent signs that North Korea intends to move forward with its nuclear weapons program. Earlier this month, satellite images suggested it restarted a key nuclear reactor at its Yongbyon site.
The moves come as the North says it is willing to resume six-nation talks on its nuclear program - talks the U.S. says can only take place if Pyongyang shows it is willing to keep its earlier promises of denuclearization.
Bruce Bennett of the RAND Corporation tells VOA, without such a commitment from the North, the U.S. feels there is no basis for negotiations.
"Very few people think the North is preparing to denuclearize. So the real question is if we do negotiate with the North, what are we going to negotiate about if they are not prepared to denuclearize?"
The U.S. has repeatedly said it will not recognize North Korea as a legitimate nuclear weapons state, even as the communist country gets closer and closer to achieving that technical status.
Bennett cautions against assuming too much about North Korean motivations. But he says as long as they insist on further developing their nuclear program, the chances for six-party talks are getting smaller and smaller.