North Korea is refurbishing its nuclear facilities and expanding its capacity to produce uranium, new satellite images by 38 North, the respected website operated by the US-Korea Institute at Johns Hopkins School of Advanced International Studies, revealed on Wednesday.
According to the 38 North report, a uranium mine and mill near Pyongsan has been under reconstruction for the last 12 months to modernize the buildings.
The report claimed that some buildings received new roofs, while others were rebuilt from scratch.
"The most recent commercial satellite imagery indicates that the Uranium Concentration Plant is currently undergoing significant refurbishment," the report said, quoted by Reuters.
"The renovation suggests that North Korea is preparing to expand the production of uranium," to "enrich the uranium to expand its stockpile of nuclear weapons," or to produce fuel for North Korea's experimental light-water nuclear reactor at its Yongbyon research plant, the report claimed.
Military drill condemned
The report came during an increased tension in the Korean Peninsula following new threats and condemnation from North Korea over a planned South Korea-US joint military exercise on Thursday.
Pyongyang described the drill as a "declaration of war" and condemned it as provocative rehearsals for a full-scale attack on the country.
The North's foreign ministry said in a statement that Washington it would be responsible for "all the consequences" if the drill is not cancelled and Pyongyang will take "all necessary measures" against US "provocations."
The official KCNA news agency also quoted a separate statement by the North's Committee for the Peaceful Reunification of Korea (CPRK), which oversees cross-border issues, saying "such large-scale joint military exercises... are little short of a declaration of a war."
The committee also threaten to strike the White House and presidential Blue House in Seoul, claiming they are "in range of the North's 'ultra-precision' military weapons," AFP reported.
Tens of thousands of troops will take part in the annual Ulchi Freedom exercise, which Washington and Seoul insist are purely defensive in nature.
This year's drill will start on Monday and it will take two weeks.
North and South Koreas exchanged war of words this week, following a recent landmine attack on border patrol in the Demilitarised Zone that maimed two South Korean soldiers. Seoul blamed North Korea over the attack, promising a strong response against "provocations" from North Korea.
South Korean officials also reported this week that North Korea's vice premier was executed by a firing squad after criticizing the policies of the country's leader, Kim Jong Un.
According to the Yonhap News Agency, the 63-year-old Choe Yong Gon was executed this year and had not been spotted in public for about eight months.
Since the 1950-1953 Korean conflict ended with a ceasfire rather than a treaty, the two Koreas are still technically at war.
North Korea is banned by the United Nations Security Council from conducting tests that use ballistic missile technology and is under heavy sanctions due to its ongoing nuclear and ballistic missile programmes.
Talks between North Korea, South Korea, the United States, Russia and Japan over the issue have been at a standstill since 2009 as the reclusive country has defied scepticism and international warnings not to pursue such a programme.