A new satellite image of a factory where North Korea makes military equipment used to launch long-range missiles has revealed the construction of a new structure.
The release of several images from Planet Labs comes amid concern that North Korea could launch a rocket or missile as it seeks concessions in stalled nuclear negotiations with the United States.
North Korea has warned that what "Christmas gift" it gives the US depends on what action Washington takes.
One of the new satellite images taken December 19 shows the March 16 Factory near Pyongyang, where North Korea manufactures trucks used as mobile launchers for its long-range missiles.
Jeffrey Lewis, director of the East Asia nonproliferation program at the Middlebury Institute, tweeted that the construction appeared to be an expansion of the factory.
Nuclear talks between Washington and Pyongyang have been stalled since a February summit between leaders Donald Trump and Kim Jong-un fell apart.
Earlier this month, North Korea carried out two major tests at its long-range rocket launch and missile engine testing site in the country's northwest.
The other images released by Planet Labs show that site before and after the December 7 test.
Meanwhile, South Korean and US special forces troops recently conducted drills simulating the infiltration of an enemy facility, US military photos seen by Reuters on Monday show, as tensions with North Korea ratchet up ahead of a year-end deadline.
South Korean and US commandos raided the facility and led out a man with his hands tied behind the back during the exercise described as a joint regular close-quarters battle training last month at a US military base in the southwestern South Korean city of Gunsan.
A South Korean official said the drill was designed as a hostage rescue operation training as part of a counter-terrorism drill conducted quarterly by the allies. The US Forces Korea did not respond to a request for comment.
The rare release of the photos by the US Defense Visual Information Distribution Service comes ahead of a year-end deadline set by Pyongyang for Washington to soften its stance in stalled talks.
The service is operated by an arm of the US Department of Defense.
South Korea's Chosun Ilbo newspaper, which first reported the training, said the drills were intended to simulate a scenario to capture North Korean executives.