On Monday, US President Donald Trump sent his first National Security Strategy report to lawmakers.
"North Korea has started experiments such as heat and pressure equipment to prevent anthrax from dying even at a high temperature of over 7,000 degrees generated at the time of ICBM's re-entry into the atmosphere", the report stated.
The source said that North Korea was likely looking to load its range of ICBMs with anthrax as its nuclear-tipped missiles can not yet reach the US mainland.
"The DPRK, as a state party to the Biological Weapons Convention (BWC), maintains its consistent stand to oppose development, manufacture, stockpiling and possession of biological weapons", North Korea's Institute for American Studies, which is affiliated with the foreign ministry, was quoted as saying by the Korean Central News Agency (KCNA). However, North Korea has set to flawless its re-entry technology.More news: Modi begins campaigning at poll-bound Meghalaya
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In response, North Korea said it would "take revenge" on the US for saying it is developing biological weapons.
The threat of an impending war with North Korea intensified after the country's state television reported last month it launched a new intercontinental ballistic missile, Hwasong 15, which is a nuclear-capable weapon that could reach the entire continental U.S.
A Japanese newspaper reports that North Korea is testing the deadly anthrax virus and other biological elements for use in delivery via long-range missile warheads.
North Korea has used chemical agents in the past to conduct attacks and assassinations, such as the February 2016 assassination of Kim Jong Un's half brother Kim Jong Nam using a VX agent at Malaysia's Kuala Lumpur International Airport.