President Donald Trump says North Korea's leader "has too much to lose" to act in a hostile way, after the secretive state's latest military test.
Kim Jong Un is believed to have ordered the "very important" test at its Sohae satellite launch site, a rocket testing ground that US officials once said the country had promised to close.
North Korean state media called it a "successful test of great significance" but did not specify what was tested.
South Korea's joint chiefs of staff, which usually issues alerts if a missile is seen launching from North Korea, declined to comment.
Speaking hours after the apparent test, Mr Trump said Mr Kim was "too smart and has far too much to lose, everything actually, if he acts in a hostile way".
"He signed a strong denuclearisation agreement with me in Singapore," the US leader also said on Twitter, referring to his first summit with Mr Kim in Singapore in 2018.
And he added Mr Kim does not want to "void his special relationship with the president of the United States or interfere" with the upcoming US election in November next year.
Missile experts said it appeared likely the North Koreans had conducted a static test of a rocket engine, rather than a missile launch.
"If it is indeed a static engine test for a new solid or liquid fuel missile, it is yet another loud signal that the door for diplomacy is quickly slamming, if it isn't already," said Vipin Narang, a nuclear affairs expert at the Massachusetts Institute of Technology.
"This could be a very credible signal of what might await the world after the New Year."
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Tensions have risen ahead of a year-end deadline set by North Korea, which has called on the US to change its policy of insisting on Pyongyang's unilateral denuclearisation and demanded relief from punishing sanctions.
On Saturday, North Korea's ambassador to the United Nations said denuclearisation was now off the negotiating table with the US and lengthy talks with Washington are not needed.
"The results of the recent important test will have an important effect on changing the strategic position of the DPRK (Democratic People's Republic of Korea) once again in the near future," North Korean state media reported.