SEOUL (Reuters) - With photographs obliquely showing a new rocket design, North Korea has sent a message that it is working on an intercontinental ballistic missile (ICBM) more powerful than any it has previously tested, weapons experts said on Thursday.

If developed, such a missile could possibly reach any place on the U.S. mainland, including Washington and New York, they said.

North Korea’s state media published photographs late on Wednesday of leader Kim Jong Un standing next to a diagram of a three-stage rocket it called the Hwasong-13.

Missile experts, who scrutinize such pictures for clues about North Korea’s weapons programs, said there is no indication the rocket has been fully developed. In any case, it had not been flight tested and it was impossible to calculate its potential range.

However, a three-stage rocket would be more powerful than the two-stage Hwasong-14 ICBM tested twice in July, they said. South Korean and U.S. officials and experts have said the Hwasong-14 may have a range of about 10,000 km (6,200 miles) and could possibly strike many parts of the United States, but not the East Coast.

“We should be looking at Hwasong-13 as a 12,000-km class ICBM that can strike all of the mainland United States,” said Kim Dong-yub, a military expert at Seoul’s Kyungnam University.

“It’s likely meant to show that they are working on a three-stage design with greater boost and range,” said retired Brigadier General Moon Sung-muk, an arms control expert who has represented South Korea in military talks with North Korea.

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He said the pictures were intended to show that North Korea was refusing to bow to international pressure to abandon its weapons programs.

“The North is trying to be in control of the playing field,” Moon said.

TENSIONS EASE

Wednesday’s report carried by the KCNA news agency lacked the traditionally robust threats against the United States, and U.S. President Donald Trump expressed optimism about a possible improvement in relations.

State Department spokeswoman Heather Nauert said it was unclear if the photos were taken before or after Secretary of State Rex Tillerson on Tuesday welcomed what he called the restraint North Korea had shown recently in its weapons programs and said he hoped a path could be opening for dialogue “sometime in the near future.”

“We consider it overall a good first step that there haven’t been any missile launches or testing for ...three-plus weeks or so,” Nauert told a regular briefing.

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However Pyongyang needed to do a lot more to show it was willing to negotiate in good faith, she said.

The photographs were accompanied by a report of Kim issuing instructions for the production of more rocket engines and warheads during a visit to the Academy of Defense Sciences, an agency he set up to develop ballistic missiles.

“We’re getting a look at it to emphasize domestic production of missiles, and to advertise what’s coming next,” said Joshua Pollack, a nuclear weapon and missile systems expert who edits the U.S.-based Nonproliferation Review.

The photographs were published as tensions between North Korea and the United States appeared to have eased slightly since North Korea tested the Hwasong-14 and later threatened to fire missiles toward the U.S. Pacific territory of Guam.

Kyungnam University’s Kim said the Hwasong-13 appeared similar to the KN-08, a three-stage missile of which only a mockup has previously been seen at military parades. But the new images show a modified design for the main booster stage that clusters two engines.

Another picture published by North Korean state media showed Kim Jong Un standing next to a rocket casing that appeared to be made of a material that could include plastic. Experts said if such material were used in the missile, it would be intended to reduce weight and boost range.

The photographs also showed the design for the Pukguksong-3, likely a new solid-fuel intermediate-range ballistic missile being developed for submarine launches.

(For a graphic on North Korean missile ranges click tmsnrt.rs/2t6WEPL)