Satellite imagery indicated on Monday that North Korea had tested a rocket engine, and a senior Pyongyang official called Donald Trump a "heedless and erratic old man", resuming insults of the U.S. president that had been set aside during a thaw.
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 towards 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."
"We have nothing more to lose."
North Korea had lauded a "special relationship" between the leaders even as it criticised other U.S. officials and their "gangster-like" demands during deadlocked talks. 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 carried out what it called a "very important" test at its Sohae satellite launching station, a rocket-testing ground that U.S. and South Korean officials once said Pyongyang had promised to shut down.
Commercial satellite images taken on Saturday by Planet Labs showed vehicles and equipment likely to be used in a rocket engine experiment, said Jeffrey Lewis, the director of the East Asia Nonproliferation Program at the Middlebury Institute of International Studies in California.
"They are mostly gone on Dec. 8, but the ground appears to have been disturbed by the exhaust from the test," Lewis said, citing photos captured on Sunday.
Asked on Monday if it had been an engine test, a spokeswoman for South Korea's defence ministry said site monitoring and detailed analysis were under way with U.S. intelligence authorities.
Kim Jong Un has warned he may take a "new path" if the United States fails to address his demands. Observers have said that path might include the launch of a space satellite, which would help North Korea demonstrate progress in its rocket capabilities without returning to overt military provocations, such as firing an intercontinental ballistic missile (ICBM).
Kim Yong Chol, the North Korean official, said Trump should try to "stop the second-hand" of a clock ticking towards conflict, instead of sticking to "bluffing and hypocrisy that sound rather abnormal and unrealistic".
"We have no intention to reconsider what we should do in the future, and will not feel worried about our future action," he said in the English-language dispatch on KCNA.
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 "successful" test of a high-power solid-fuel rocket engine. While inspecting a new missile modelled upon SLBMs in February 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."