One of 20 undeclared ballistic missile operating bases in North Korea serves as a missile headquarters, according to a report by the Center for Strategic and International Studies (CSIS) a Washington-based think-tank published on Monday.

"The Sino-ri missile operating base and the Nodong missiles deployed at this location fit into North Korea's presumed nuclear military strategy by providing an operational-level nuclear or conventional first strike capability," said the report co-authored by analyst Victor Cha.

The discovery of an undeclared missile headquarters comes three days after US President Donald Trump said he "looks forward" to another summit to discuss denuclearisation with North Korean leader Kim Jong Un in late February.

The Media is not giving us credit for the tremendous progress we have made with North Korea. Think of where we were at the end of the Obama Administration compared to now. Great meeting this week with top Reps. Looking forward to meeting with Chairman Kim at end of February! — Donald J. Trump (@realDonaldTrump) January 20, 2019

Kim vowed to work towards denuclearisation at his first summit with Trump in June, but there has since been little concrete progress.

The White House did not immediately respond to a request for comment.

CSIS, which last reported on the 20 missile bases in November, said the Sino-ri base has never been declared by North Korea and as a result "does not appear to be the subject of denuclearisation negotiations."

The report said that missile operating bases would presumably be subject to declaration, verification, and dismantlement in any nuclear deal.

"The North Koreans are not going to negotiate over things they don't disclose," said Cha.

"It looks like they're playing a game. They're still going to have all this operational capability," even if they destroy their disclosed nuclear sites.

Satellite imagery from @BeyondCSISKorea confirms that Sino-ri - the oldest of approximately 20 undeclared missile operating bases - houses a regiment-sized unit equipped with Nodong-1/-2 medium-range ballistic missiles (MRBM).



Read more: https://t.co/bmZIeh6TVG pic.twitter.com/HfswREkEw6 — CSIS (@CSIS) January 21, 2019

Sino-ri complex

Located 212km north of the demilitarised zone, the Sino-ri complex is an 18sq-km base that plays a key role in developing ballistic missiles capable of reaching South Korea, Japan, and even the US territory of Guam in the western Pacific, the report said.

It houses a regiment-sized unit equipped with Nodong-1 medium-range ballistic missiles, the report added.

Satellite images of the base from December 27, 2018 show an entrance to an underground bunker, reinforced shelters and a headquarters, the report said.

In South Korea, the Sino-ri facility has long been known as one of the bases housing the Nodong, also called the Rodong, a medium-range missile based on Soviet-era Scud technology that North Korea began deploying in the mid-1990s.

"It is a facility we've been monitoring with interest, in cooperation with the United States," Kim Joon-rak, a spokesman for South Korea's Joint Chiefs of Staff, told a news briefing on Tuesday.