WASHINGTON >> Commercial satellite imagery shows increased activity at North Korea’s nuclear test site but not enough to indicate an underground atomic explosion is imminent, a U.S. research institute said Tuesday.

North Korea last month threatened to conduct its fourth nuclear test and there’s been speculation it may do so as President Barack Obama travels to Asia this week.

U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said that recent images, the latest taken Saturday, appear to show movement of crates and possibly lumber near tunnel entrances at the northeastern mountain site of Punggye-ri. But it said in an analysis published on its website — 38 North — that more movements of vehicles and equipment were detected in the weeks before previous detonations.

South Korea’s Defense Ministry also said Tuesday it has detected "various activities" at Punggye-ri, where North Korea has conducted three nuclear tests since 2006, the latest in February 2013.

In Seoul, ministry spokesman Kim Min-seok told reporters that the South Korea military was keeping in mind that North Korea can conduct a nuclear test suddenly, or could be just pretending to prepare for one "and deceive us, like they did in the past."

In Washington, State Department spokeswoman Jen Psaki said the U.S. was closely monitoring the situation on the Korean Peninsula and continued to urge North Korea "to refrain from actions that threaten regional peace and security." The Pentagon said it could not confirm the reports that a nuclear test was being prepared.

It is notoriously difficult to divine the intentions of North Korea’s isolated regime, particularly on nuclear tests, as the most crucial activity happens underground and out of aerial view. 38 North also cautions that commercial satellite imagery is relatively infrequent so it provides only a snapshot of what’s going on at Punggye-ri.

Tensions on the Korean Peninsula have escalated over the past month, as North Korea has objected strongly to the U.S. and South Korea conducting annual military exercises that ended last week. The North has test-fired two medium-range ballistic missiles and exchanged artillery fire with South Korea at their sea border.

In a briefing ahead of Obama’s departure Tuesday on his four-nation trip, that will include South Korea, the White House senior director for Asian affairs, Evan Medeiros, told reporters: "Right now we are going into an environment where there’s a growing number of threats and risks of provocation." He said North Korea’s threat to conduct a nuclear test showed it wasn’t interested in credible negotiations.

Another test explosion would deepen international concern and crank up international pressure on North Korea, and doubtless anger and embarrass the North’s only major ally, China. Washington and its allies would push to tighten U.N. sanctions against Pyongyang.

In its analysis, 38 North said that in addition to the materials seen outside two tunnel entrances in the south of the site, over the past six weeks, there’s been an uptick in activities at a support area at Punggye-ri that was used for managing operations for the last test. An April 19 image also shows a large trailer truck traveling down the road away from the test site.

"Based on available information, activities at the Punggye-ri test site could represent an early stage of preparations for a test or may be intended for a less dangerous purpose, to conduct maintenance with the end of winter," 38 North says.