Image copyright Getty Images Image caption North Korea says recent missile launches are a warning to the US and South Korea

North Korea has stolen $2bn (£1.6bn) to fund its weapons programme using cyber-attacks, a leaked United Nations report says.

The confidential report says Pyongyang has targeted banks and crypto-currency exchanges to collect cash.

Sources confirmed to the BBC that the UN was investigating 35 cyber-attacks.

North Korea launched two missiles on Tuesday, the fourth such launch in less than two weeks.

In a statement on Wednesday, the North's leader Kim Jong-un said the launches were a warning against joint military exercises being carried out by the US and South Korea.

Pyongyang has described the exercises as a violation of peace agreements.

The leaked report, sent to the UN Security Council's North Korea sanctions committee, says Pyongyang "used cyber-space to launch increasingly sophisticated attacks to steal funds from financial institutions and crypto-currency exchanges to generate income".

Experts are also investigating cyber-mining activity - the use of powerful computers to generate virtual currency - designed to earn foreign money.

The report adds that North Korea's attacks against crypto-currency exchanges allowed it to "generate income in ways that are harder to trace and subject to less government oversight and regulation than the traditional banking sector".

The report also says North Korea has violated UN sanctions by means of illicit ship-to-ship transfers, as well as obtaining items related to weapons of mass destruction.

Since 2006, the UN Security Council has imposed sanctions on North Korea that ban exports including coal, iron, lead textiles and seafood. There has also been a cap on imports of crude oil and refined petroleum products.

Kim Jong-un agreed during a summit with US President Donald Trump in Singapore last year that he would stop nuclear testing.

Mr Kim also agreed that North Korea would no longer launch intercontinental ballistic missiles.

A second summit between the two leaders in Hanoi in 2019 ended without agreement.

Since then, denuclearisation talks have stalled although both sides say they still want to pursue diplomacy.

In response to the UN report, a US state department spokeswoman told Reuters news agency: "We call upon all responsible states to take action to counter North Korea's ability top conduct malicious cyber activity, which generates revenue that supports its unlawful WMD and ballistic missile programmes."