Biegun is scheduled to hold talks on Thursday with his South Korean counterpart, Lee Do-hoon.
The warning from the Pyongyang came days after the US said it was imposing sanctions on three senior North Korean officials over human rights abuses.
The North's stinging response came after the United States said it had introduced sanctions on three North Korean officials.
Kim's first observation of a weapons test this year could complicate already stalled nuclear talks with the United States.
It marked the first official report of a weapons test by North Korea since it began a delicate diplomatic process with Washington over its nuclear and missile programme.
The US-China Economic and Security Review Commission says China appears to have relaxed enforcement of sanctions on North Korea.
The facilities identified in the new report are located in strategic locations that would put missiles in range of South Korea and Japan.
The present comes after the North Koreans gave South Korean President Moon Jae-in two tonnes of pricey pine mushrooms.
A planned meeting between US Secretary of State Mike Pompeo and top North Korean official Kim Yong Chol to push for progress on denuclearisation was cancelled at Pyongyang's request.
Pompeo said Kim told him he was committed to the promises he had made to Trump during their first summit.
The talks were the first meeting of a trilateral JSA commission made up of the two Koreas and the UN Command, which is included as it retains jurisdiction over the southern half of the JSA.
Moon, who has met Kim Jong Un three times this year, was set to convey the message to French President Emmanuel Macron in Paris on Monday as part of a seven-day European tour.
The North has stopped all nuclear and missile tests, dismantled its only nuclear test site and is now dismantling its missile engine test facilities, and promises to dismantle its Yongbyon nuclear complex if the US takes corresponding measures.
Pompeo also says both sides are 'pretty close' to agreement on the details of a second summit, which Kim proposed to US President Donald Trump in a letter last month.
Pompeo's trip to Pyongyang occurred amid negative signals from North Korea whose actions have fallen short of Washington's demands for a complete inventory of its nuclear weapons and irreversible steps to give up an arsenal that potentially threatens the United States.
Pompeo and Kim also agreed to form a working group to discuss the denuclearisation process and the second summit,
Pompeo had planned to travel to North Korea in August but Trump canceled the trip at the last moment and publicly acknowledged for the first time that his efforts to get Pyongyang to denuclearise had stalled.
The North has for decades demanded that the US formally declare the end of the 1950-53 conflict that was halted only with an armistice, saying an official end to the war would ease tensions on the flashpoint peninsula.
Both sides undertook to carry out landmine removal work at the Joint Security Area (JSA) over a 20-day period, according to the South Korean defence ministry.
While Moon said it’s natural to be suspicious of Kim’s motivation given the North’s past failures to live up to its denuclearisation agreements, he added that this time it was 'completely different.'
During a meeting with South Korean President Moon Jae-in at the United Nations, Trump says Kim has been 'really very open and terrific, frankly.'
While relations with Kim have improved dramatically, leaders attending the annual assembly will hear how another of Trump's adversaries, Iran's Hassan Rouhani, remains beyond the pale for the American president.
Pyongyang is believed to have produced its plutonium at Yongbyon, which is also known to house a uranium enrichment plant.
The process had become deadlocked until Moon's trip to Pyongyang, where Kim agreed to permanently dismantle a missile testing site.