A new satellite image of a factory where North Korea makes military equipment used to launch long-range missiles shows 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.

As unease over Pyongyang's intentions mounted, leaders from China, Japan and South Korea reiterated their commitment to ending North Korea's nuclear and missile programs at a trilateral summit Tuesday. It came against the backdrop of Pyongyang's increasing demands for sanctions relief.

This Dec. 19, 2019, satellite image from Planet Lab Inc., that's been analyzed by experts at the Middlebury Institute of International Studies shows the March 16 Factory in Pyongsong, near Pyongyang, where North Korea manufactures military trucks used as mobile launchers for long-range missiles. This new satellite image on a North Korean missile-related site shows the construction of a new structure this month. Planet Labs Inc., Middlebury Institute of International Studies via AP

Free trade and economic cooperation were also at the forefront of the meetings between Chinese Premier Li Keqiang, Japanese Prime Minister Shinzo Abe and South Korean President Moon Jae-in in the southwestern Chinese city of Chengdu.

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CBS News National Security correspondent David Martin reports that while a short-range missile test or rocket engine test by North Korea could come at any time, a long-range intercontinental ballistic missile (ICBM) test – although possible – isn't expected until after the first of the year. Martin cites U.S. intelligence officials.

It's traditional for North Korean leader Kim Jong Un to deliver a New Year's address, during which he generally lays out the strategic focus for the year and any major shifts in policy or engagement. North Korea experts will be watching to see whether Kim uses the address to declare the diplomatic window with the U.S. is now closed and then takes action by launching an ICBM. Or he could launch one between now and then and use the speech to explain why the test was carried out.

Kim has set a deadline for the end of the year for the U.S. to offer terms for a nuclear deal, and said this month that it's up to the U.S. to choose which "Christmas gift" it receives from the North.

One of the satellite images taken on Dec. 19 shows the completion of a new structure at the March 16 Factory near Pyongyang, where North Korea is believed to be developing and manufacturing vehicles used as mobile launchers for long-range ballistic missiles.

Jeffrey Lewis, director of the East Asia nonproliferation program at the Middlebury Institute, said in an email that the construction seemed to be an expansion of the factory, which would be "big news."

North Korea used what appeared to be vehicles imported from China during its three flight tests of the Hwasong-14 and -15 intercontinental ballistic missiles in 2017, Lewis said. An increased capacity to produce mobile launchers would potentially help expand North Korea's ICBM force and improve its survivability during nuclear conflict.

Lewis said, as far as he knows, North Korea only imported six of the vehicles that were used during the 2017 ICBM tests.

"I would think North Korea would want 50-100 such systems," he said. "This would probably be some mix of ICBMs we have seen and the new system that North Korea claims is under development."

Nuclear talks between the U.S. and North Korea have been stalled since a February summit between President Trump and North Korean's Kim fell apart.

Earlier this month, Pyongyang carried out two major tests at its long-range rocket launch and missile engine testing site in the country's northwest. Experts believe it tested a new engine for either an ICBM or a satellite launch vehicle.

The other images released by Planet Labs show that site before and after the Dec. 7 test.