North Korea has conducted three underground nuclear tests, triggering United Nations sanctions, in 2006, 2009 and last February, all at the Punggye-ri site. The yield of the detonations has increased with each test, but international experts have disputed North Korea’s repeated implication that it has succeeded in making its bombs small and sophisticated enough for missile delivery. North Korea is also attempting to develop intercontinental ballistic missiles.

Speaking last month at a forum at the Asan Institute for Policy Studies in Seoul, Li Bin, a Chinese arms control expert at the Carnegie Endowment for International Peace, said he believed that North Korea had detonated a miniaturized nuclear device in its first test. But after that detonation failed to reach a normal yield, he said, the country had to increase the size of its bombs in order to boost the yield in the subsequent tests. “The size of its reliable device today is still too big for missile delivery,” he said.

On Thursday, the U.S.-Korea Institute said that satellite photos as recent as Sept. 27 showed two new tunnel entrances at Punggye-ri, as well as growing piles of dirt. This suggested that the North might be digging new tunnels for nuclear tests, or it might be building a connection to existing tunnels, the report said. The North also appeared to be upgrading its support facilities in the area, the institute said.

These activities “indicate that North Korea is planning to conduct future detonations as part of its overall nuclear weapons development program,” the institute said in its report, posted on its Web site, 38 North.

South Korean officials have said that the North might conduct another nuclear test, whether to advance its weapons program or to raise tensions as a bargaining tactic, but they have also said that it is difficult to predict when the North might do so. While satellite images of the North’s nuclear sites are often scrutinized by analysts, the images cannot reveal what is happening within the tunnels and buildings themselves, and Pyongyang is sometimes suspected of increasing activity at the sites simply to put pressure on the United States to negotiate.