UPDATE 7-Trump says Kim Jong Un risks losing 'everything' after North Korea claims major test

* Reported test comes ahead of year-end N.Korean deadline

* Sohae test site was partially dismantled then rebuilt

* Analysts had noted activity at Sohae recently, suggesting test imminent (Adds comments from U.S. national security adviser)

By Soyoung Kim and Josh Smith

WASHINGTON/SEOUL, Dec 8 (Reuters) - U.S. President Donald Trump said on Sunday that North Korean leader Kim Jong Un risks losing "everything" if he resumes hostility and his country must denuclearize, after the North said it had carried out a "successful test of great significance."

"Kim Jong Un is too smart and has far too much to lose, everything actually, if he acts in a hostile way. He signed a strong Denuclearization Agreement with me in Singapore," Trump said on Twitter, referring to his first summit with Kim in Singapore in 2018.

"He does not want to void his special relationship with the President of the United States or interfere with the U.S. Presidential Election in November," he said.

North Korea's state media KCNA reported earlier on Sunday that it had carried out a "very important" test at its Sohae satellite launch site, a rocket-testing ground that U.S. officials once said North Korea had promised to close.

The reported test comes ahead of a year-end deadline North Korea has imposed for the United States to drop its insistence on unilateral denuclearization. Pyongyang has warned it could take a "new path" amid the stalled talks with the United States.

"North Korea, under the leadership of Kim Jong Un, has tremendous economic potential, but it must denuclearize as promised," Trump said on Twitter.

The KCNA report called it a "successful test of great significance" but did not specify what was tested.

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 in the United States.

"This could be a very credible signal of what might await the world after the New Year."

Tensions have risen ahead of a year-end deadline set by North Korea, which has called on the United States to change its policy of insisting on Pyongyang's unilateral denuclearization and demanded relief from punishing sanctions.

On Saturday North Korea's ambassador to the United Nations said denuclearization was now off the negotiating table with the United States and lengthy talks with Washington are not needed.