North Korea has carried out a "very important" test at its Sohae satellite launch site, state media KCNA reported on Sunday, a rocket testing ground that US officials once said Pyongyang had promised to close.
The reported test comes as a year-end deadline North Korea has imposed nears, warning it could take a "new path" amid stalled denuclearisation talks with the United States. The KCNA report called it a "successful test of great significance" on Saturday but did not specify what was tested.
South Korea's defence ministry said South Korea and the United States are cooperating closely in monitoring activities at major North Korean sites including Tongchang-ri, the area where Sohae is located.
Missile experts said it appeared likely the North Koreans had conducted a static test of a rocket engine, rather than a missile launch, which are usually quickly detected by neighbouring South Korea and Japan.
"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 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 United States 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 once again in the near future," KCNA reported, using the initials of North Korea's official name, the Democratic People's Republic of Korea.