A report on the investigations was released by United Nations panel of experts dated September 9.
The report was issued two days prior to the UN Security Council's unanimous adoption of a tough new round of sanctions aimed at coercing North Korea into negotiations on its nuclear arsenal.
The eight-member panel said it is assessing information from an unnamed UN member-state indicating that Tanzania has entered into military-related contracts with a North Korean corporation valued at about $12.5 million.
The Haegeumgang Trading Corporation is said to be repairing and upgrading Tanzanian surface-to-air missile systems and air defence radar.
“The United Republic of Tanzania is yet to respond to the panel’s enquiries,” the UN investigators said.
The panel added that it has continued its investigation into North Korean training of Ugandan military and police forces, “in particular the Ugandan air force.”
Also under investigation are the activities of a representative of the Korea Mining Development Trading Corporation, who had travelled to Uganda from Syria.
In addition, the panel said it is looking into the role of the military attaché office in the North Korean embassy in Kampala.
The report indicates that Uganda has also not responded to its queries.
The 111-page document makes no reference to an announcement last year by South Korean officials that Uganda had agreed to cut its military and police ties with North Korea.
President Yoweri Museveni made that pledge during a meeting with his South Korean counterpart in May 2016, the office of South Korea's president said at the time.
Ugandan Foreign Minister Sam Kutesa subsequently told NBS Television, “We are disengaging the cooperation we are having with North Korea as a result of UN sanctions.”
Last week's UN panel report said "lax enforcement" of sanctions has allowed North Korea to earn $270 million in foreign transactions since February this year.
North Korea has woven a web of "evolving evasion techniques" that are undermining the goals of UN Security Council resolutions, the report added.
Nikki Haley, US ambassador to the UN recently warned that the United States is considering restricting trade with countries that violate sanctions by doing business with North Korea.
Angola, the Democratic Republic of Congo, Eritrea, Mozambique and Namibia are also named by the panel of experts as subjects of UN investigation regarding sanctions violations.
Other African countries under probe are Angola, Benin, Botswana, Mali, Mozambique, Namibia and Zimbabwe for contracting North Korea's Mansudae Overseas Project Group for services, including provision of statues.
North Korea has had friendly relations with some African countries since the anti-colonial struggles of the 1960s, which it supported.