North Korea has been restoring part of the Dongchang-ri nuclear missile launch site in Yongbyon, north of Pyongyang, the National Intelligence Service, South Korea's chief intelligence agency said on Tuesday.
"(The North) appears to be putting back a roof and a door (to a Dongchang-ri facility)," South Korean news agency, Yonhap reported quoting the NIS.
North Korea had stopped the nuclear reactor, located within the nuclear complex in Yongbyon, north of Pyongyang, late last year, with no signs of reprocessing activities there anymore.
In addition, the underground tunnels of the nuclear test site remained shut and unattended since their destruction in May last year.
The NIS further noted that the army of South Korea along with the United States has run a "thorough" monitoring system to keep track of the North's nuclear and missile facilities, including its uranium enrichment sites.
"The US information is the same as ours, but we can't comment on what facilities are located where," the NIS was quoted as saying during a press briefing.
The 5-megawatt graphite-moderated reactor is capable of producing spent fuel rods, which if reprocessed can yield five to seven kilograms of weapons-grade plutonium annually. About 6 kg of plutonium is required to build a single bomb.
The Yongbyon complex became a part of key agendas related to the denuclearisation talks during the latest summit between US President Donald Trump and North Korean leader Kim Jong-un that took place in Hanoi last month. However, both the leaders had moved out of the meeting without reaching an agreement as they failed to reach a consensus over the denuclearisation of the Korean peninsula.
The US and North Korea had reached an impasse regarding sanctions relief for Pyongyang soon after the first Summit in June 2018, leading to extremely slow progress on the denuclearisation front.
North Korea has sought relief in sanctions in light of the steps taken by the country towards denuclearisation. The US, however, has repeatedly stated that there would be no sanctions relief until complete denuclearisation is achieved by North Korea.