Pyongyang has accused the United States of using “biological warfare schemes” to target North Korea with live anthrax and wants the United Nations Security Council (UNSC) to investigate.

The accusations were made in a letter to the UNSC and Secretary General Ban Ki-Moon from North Korea’s ambassador to the UN Ja Song Nam.

The letter, which was dated June 4, was made public on Friday and comes after US defense officials revealed in late May that low concentration samples of live anthrax were mistakenly shipped to labs in 19 states and to at least 70 labs in four countries, including a US facility based in South Korea. Australia, Britain, and Canada have also reported receiving shipments of the deadly bacteria.

According to the US Defense Department, the anthrax was supposed to have been killed before it was shipped to laboratories for research, a practice that has been carried out successfully for ten years.

"The United States not only possesses deadly weapons of mass destruction…but also is attempting to use them in actual warfare against [North Korea]," Nam wrote in his letter.

Referring to the delivery as "the gravest challenge to peace and a hideous crime aimed at genocide," the letter also "strongly requests the Security Council take up the issue of the shipment of anthrax germs in order to thoroughly investigate the biological warfare schemes of the United States."

Pyongyang has been openly critical of the US’ military presence in South Korea, and has strongly objected annual US-South Korean military exercises which it views as a threat.

State Department spokesman Jeff Rathke dismissed the allegations on Friday, telling reporters in Washington that they were "ridiculous" and "didn’t merit a response."

The US House of Representatives announced on Wednesday that a thorough investigation has been launched into the incident, and vowed that they will hold the Defense Department accountable for the error.