North Korean state media released photos Wednesday that appear to show concept designs for at least one new, more advanced missile.

The diagrams were spotted hanging on a wall behind leader Kim Jong Un as he visited a plant that makes solid-fuel engines for the country’s ballistic missile program.

One of the diagrams showed a missile called “Pukguksong-3” that is apparently the latest in the Pukguksong, or Polaris, series. The other photo, which is harder to discern, appears to show a “Hwasong,” or Mars missile.

The images, which were released by the official Korean Central News Agency (KCNA), were published in the ruling Workers' Party’s Rodong Sinmun newspaper.

The KCNA said Kim called on workers at the plant to produce more solid-fuel rocket engines and rocket warhead tips.

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Michael Duitsman, a research associate at the Center for Non-Proliferation Studies, told the Associated Press that the first missile had not been seen before.

“The Pukguksong-3 is definitely new,” he said in an email.

Last month, North Korea test-fired two Hwasong-14 intercontinental ballistic missiles it claims are capable of hitting the U.S. mainland.

The diagrams come amid high tensions between the U.S. and North Korea over the North's nuclear ambitions.

On Tuesday, President Trump’s administration imposed sanctions on 10 companies and six individuals from mostly China and Russia who it says helped North Korea develop its nuclear and ballistic missile programs.

It came the day after the U.S. and South Korea started their annual joint military exercises amid threats from Pyongyang that the drills could spark a “second Korean war.”

The 11-day exercise will involve 17,500 American troops and roughly 50,000 South Korean service members, according to U.S. and South Korean authorities. North Korea sees the drills as a preparation for war.

Kim and Trump exchanged a war of words earlier this month amid a later-deferred plan by Pyongyang to fire four ballistic missiles in the direction of the U.S. territory of Guam.

Contributing: The Associated Press