The UN is investigating at least 35 reported instances of Pyongyang "attacking financial institutions, cryptocurrency exchanges and mining activity designed to earn foreign currency," it said.

"Large-scale attacks against cryptocurrency exchanges allow the DPRK to generate income in ways that are harder to trace and subject to less government insight and regulation than the traditional banking sector," the report added.

North Korea has launched four pairs of projectiles in less than two weeks and threatened more, amid fears it is ramping up its missiles program.

Leader Kim Jong Un says the country's latest missile launches on Tuesday were a warning to Washington and Seoul over their joint war games, state news agency KCNA reported, as tensions rise on the Korean peninsula.

"Cyber actors, many operating under the direction of the Reconnaissance General Bureau (North Korea's intelligence agency), raise money for its WMD (weapons of mass destruction) programmes, with total proceeds to date estimated at up to two billion US dollars," said the UN report.

It added that North Korea is violating UN sanctions by buying luxury goods and equipment related to weapons development through "ongoing illicit ship-to-ship transfers."

"These and other sanctions are facilitated through the DPRK's access to the global financial system, through bank representatives and networks operating worldwide.

"The DPRK has also used cyberspace to launch increasingly sophisticated attacks to steal funds from financial institutions and cryptocurrency exchanges to facilitate income," said the report.

The study, prepared for the UN Security Council's North Korea sanctions committee, also highlighted intelligence showing that Pyongyang had transferred petroleum and coal in violation of resolutions.

The Security Council has imposed a number of sanctions on North Korea since the country conducted its first nuclear test in 2006.

Pyongyang and Washington are engaged in a long-running diplomatic process over the North's nuclear and missile programs that has seen US President Donald Trump and Kim hold two historic summits.

The North made a vague pledge on denuclearization during the first meeting in June last year.

A second summit in Hanoi this February broke up amid disagreements over sanctions relief and what Pyongyang might be willing to give up in return.

The two agreed to resume nuclear talks during their impromptu June meeting in the Demilitarized Zone that divides the peninsula, but working-level dialogue has yet to begin.