SEOUL (Reuters) - A senior North Korean official called Donald Trump a “heedless and erratic old man” on Monday, resuming insults of the U.S. president that had been set aside during a thaw.

FILE PHOTO - A North Korean flag flies on a mast at the Permanent Mission of North Korea in Geneva October 2, 2014. REUTERS/Denis Balibouse/File Picture

The new insults came a day after Pyongyang announced a “very important” test of an unspecified device. Satellite imagery indicated on Monday it may have been a rocket engine, an indication of possible future plans if Pyongyang carries out threats to declare disarmament talks a failure at year’s end.

The statement carried in state media KCNA by Kim Yong Chol, a ruling party vice chairman who was instrumental in arranging a failed second summit in February, was the strongest salvo yet in a war of words that has rekindled in recent days.

He described Trump as impatient, rebuked him over his own rhetoric and repeated a threat from last week that Pyongyang would dust off its previous insult “dotard” for the U.S. leader.

Tensions have been rising in recent weeks as a year-end deadline approaches set by North Korea for Washington to soften its stance in negotiations. Pyongyang has conducted a series of weapons tests and issued strongly worded statements.

Though Trump regularly exchanged insults with North Korea in the first years of his term, both sides had abandoned personal attacks after Trump met North Korean leader Kim Jong Un in Singapore in 2018.

The North Korean official said the country’s leader may change his views toward Trump if the president continues uttering “inappropriate, highly risky words and expressions”.

He pointed to remarks by Trump on Sunday that Kim had “far too much to lose” and did not want to interfere with an upcoming U.S. presidential election.

Those comments indicate Trump is “an old man bereft of patience”, the North Korean official said. “As he is such a heedless and erratic old man, the time when we cannot but call him a ‘dotard’ again may come.”

Another senior North Korean official, Ri Su Yong, issued a separate statement saying Trump should stop using abusive language that may further offend Kim.

Trump should think twice if he does not want to see “bigger catastrophic consequences”, said Ri, a vice chairman of the ruling Workers’ Party Korea, adding that North Korea will make a final judgment by the end of this year.

North Korea had lauded a “special relationship” between the leaders even as it criticized other U.S. officials. However, Pyongyang bristled last week after Trump again referred to Kim as “Rocket Man”, a nickname Trump used early in his term.

On Sunday North Korea said it carried out a “very important” test at its Sohae satellite launching station.

Jeffrey Lewis, director of the East Asia Nonproliferation Program at the Middlebury Institute of International Studies in California, said satellite pictures of the site showed vehicles and equipment likely to be used in a test of a rocket engine, which were visible on Saturday but no longer present on Sunday.

“The ground appears to have been disturbed by the exhaust from the test,” he added.

South Korea’s defense ministry said the site was being monitored and analyzed along with U.S. intelligence authorities.

SOLID FUEL

Kim Jong Un has warned he may take a “new path” if the United States fails to address his demands. Some experts say that might include the launch of a space satellite, which would help North Korea demonstrate progress in rockets without returning to overt military provocations, such as firing an intercontinental ballistic missile (ICBM).

Some South Korean experts said North Korea may have tested a solid fuel rocket engine, which could allow it to field ICBMs that are easier to hide and faster to deploy.

“They may well have tried to see the thrust and duration of a solid-propellant rocket engine for ICBMs,” a diplomatic source in Seoul told Reuters. “That’s effectively what they can do on the ground at this point without firing anything into the air.”

North Korea appears to have used Soviet-era liquid propellants in all its ICBM or satellite launches in recent years, while developing submarine-launched ballistic missiles (SLBMs) based on solid fuel, officials and analysts say.

In March 2016, Kim oversaw a test of a high-power solid-fuel rocket engine. While inspecting a new missile modeled upon SLBMs in 2017, he said the country’s rocket industry had “firmly transitioned” to solid fuel from liquid propellants.

“It could be solid fuel or they might have developed a new engine,” said Jeong Han-beom, director of the Graduate School of National Security at Korea National Defense University.

“In any case, it’s meant to improve their capabilities for ICBMs, which need to be tested several times, while sending a message to Washington that we might go back to those old days of military confrontation if negotiations fail.”