Washington: Experts have revealed an undeclared site that reportedly serves as the headquarters of one of North Koreas ballistic missile programmes, the media reported on Tuesday.

The Sino-ri site, one of 20 North Korea is suspected of failing to declare, houses medium-range Nodong missiles that could be used in nuclear or conventional attacks on South Korea, Japan and the US territory of Guam, according to a report by the Centre for Strategic and International Studies (CSIS) in Washington.

"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," the Guardian quoted the report, co-written by analyst Victor Cha, as saying on Monday night.

Located 212 km north of the demilitarized zone separating the two Koreas, the Sino-ri complex is a 18 sq km base that houses a regiment-sized unit equipped with Nodong-1 medium-range missiles, it said.

The discovery has called into question North Korea's intentions as officials met in Sweden this week to discuss the arrangements for leader Kim Jong-un's second meeting with US President Donald Trump, which is expected to take place next month, possibly in Vietnam.

The leaders have made little progress since Kim signed a vaguely worded statement committing him to the denuclearization of the Korean peninsula in exchange for security guarantees from Washington during their first summit, in Singapore last June.

Monday's report said the base had never been declared by the North and as a result "does not appear to be the subject of denuclearization negotiations".

Noting that missile bases would presumably have to be part of any agreement committing North Korea to "complete, verifiable, and irreversible" denuclearization, Cha warned: "The North Koreans are not going to negotiate over things they don't disclose. It looks like they're playing a game."

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