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.

Building up to a launch

The test is the latest in a string of statements and actions from North Korea designed to underscore the seriousness of its deadline.

North Korea has announced it would convene a rare gathering of top ruling-party officials later this month, and on Wednesday state media showed photos of leader Kim Jong Un taking a second symbolic horse ride on the country's sacred Mt. Paektu.

Such meetings and propaganda blitzes often come ahead of major announcements from North Korean authorities.

While North Korea has not specified what its “new path” could be, observers have suggested the launch of a space satellite is a possibility, allowing Pyongyang to demonstrate and test its rocket capabilities without resorting to overt military provocation such as an intercontinental ballistic missile (ICBM) launch.

“Such testing is meant to improve military capabilities and to shore up domestic pride and legitimacy,” Leif-Eric Easley, a professor at Ewha University in Seoul, said of Saturday's test.

“North Korea is avoiding violations of its long-range missile test moratorium for now, but it is still improving the propulsion and precision of its missiles so that it can claim a credible nuclear deterrent,” he said.

Kim Dong-yub, a former South Korean Navy officer who teaches at Kyungnam University in Seoul, said North Korea may have tested a solid fuel rocket engine, which could allow North Korea to field ICBMs that are easier to hide and faster to deploy.

“North Korea has already entered the new path that they talked about,” he said.

'Reversible steps'

US President Donald Trump told reporters in June 2018 after his first summit with North Korean leader Kim Jong Un that North Korea had pledged to dismantle one of its missile installations, which US officials later identified as Sohae.

Shortly after that summit, analysts said satellite imagery showed some key facilities at Sohae being dismantled.

However, in the wake of the second summit between Trump and Kim earlier this year, which ended with no agreement, new imagery indicated the North Koreans were rebuilding the site.

At the time Trump said he would be “be very disappointed” if the reports of rebuilding were true.

“Remember this is at the site that was supposedly dismantled as a 'denuclearisation step,'” Narang said. “So this is a first step at 'renuclearising.' Reversible steps are being...reversed.”

In recent weeks, media reports indicated a high number of US military surveillance flights over the Korean peninsula, suggesting growing expectation of North Korean tests.

Commercial satellite imagery captured on Thursday by Planet Labs showed new activity at the Sohae Satellite Launching Station and the presence of a large shipping container, CNN reported, with analysts suggesting it indicated a test was imminent.