North Korea appears to have the capability to conduct another nuclear test whenever it chooses, a U.S. institute monitoring the country said Friday.
"It is highly likely" that the Punggye-ri test site in the northeast "is capable of supporting additional tests at any time," the U.S.-Korea Institute of Johns Hopkins University said on its 38 North website, showing commercial satellite imagery taken on Monday.
The country has conducted four nuclear tests, the most recent one in January.
The analysis came after North Korean state media quoted Kim Jong Un on Tuesday as saying his country will soon conduct more nuclear warhead and missile tests to improve its offensive capabilities.
North Korea recently launched short- and medium-range ballistic missiles despite a tightening of sanctions on the country under a U.N. Security Council resolution passed March 2 in the wake of its fourth nuclear test, as well as a rocket launch in February. The resolution demanded that Pyongyang refrain from such acts.
The institute said that the photos indicate continued activity at the test site that "does not appear directed at further tunnel excavation but rather to maintain existing tunnels as well as to clean up" after the Jan. 6 nuclear test.
The institute said the nuclear test was conducted at the North Portal of the test site and the facility "remains active with footpaths around the buildings and what appears to be water draining from the entrance along the drainage ditch."
"This may support the hypothesis that branching tunnels lead to other test chambers since the shaft used for the January test should have been completely sealed to prevent radioactivity leakage," it said.