North Korea attempted at least seven attacks on cryptocurrency platforms last year, netting about $US400 million ($458 million) in digital assets, according to new research from blockchain monitoring firm Chainalysis.

"From 2020 to 2021, the number of North Korean-related hacks increased from four to seven, while the value extracted from these hacks increased by 40%," the report, released on Friday AEST, stated.

"Once North Korea gained custody of the funds, they immediately began a meticulous money laundering operation to conceal and cash out," the report continued.

A United Nations panel of experts that monitors North Korea's sanctions has accused Pyongyang of evading restrictions by using stolen funds to bolster its nuclear and ballistic missile programs.

North Korea does not reply to media requests but has previously issued comments refuting hacking charges.

The US charged three North Korean computer programmers working for the country's intelligence service last year with a significant, years-long hacking spree to steal more than $US1.3 billion in cash and cryptocurrency from organizations ranging from banks to Hollywood film studios.

Chainalysis did not identify all of the hackers' targets. Still, they stated that the majority were investment firms and centralized exchanges, including Liquid.com, which announced in August that an unauthorized user gained access to some of the cryptocurrency wallets it managed.

According to the report, the attackers used phishing lures, code exploits, malware, and advanced social engineering to divert funds from these organizations' internet-connected 'hot' wallets to North Korea-controlled addresses.

Many of last year's attacks were almost certainly carried out by the Lazarus Outfit, a US-sanctioned hacking group that claims to be directed by North Korea's Reconnaissance General Bureau, the country's top intelligence agency.

The organization has been charged with being involved in the "WannaCry" ransomware attacks, hacking multinational banks and customer accounts, and the 2014 cyberattacks on Sony Pictures Entertainment.

North Korea also looked to have increased its efforts to launder stolen cryptocurrency, considerably boosting its use of mixers or software tools that pool and scramble money from thousands of addresses, Chainalysis reported.

According to the paper, analysts discovered $US170 million in unlaundered bitcoin holdings from 49 distinct attacks between 2017 and 2021.

The report stated that while it is unknown why the hackers have retained these funds, they may be hoping to outwit law enforcement interest before cashing out.

"Whatever the reason, the length of time that (North Korea) is willing to retain these funds is instructive, as it indicates a deliberate strategy, not a hasty one," Chainalysis said.

On Friday, North Korea chastised the Biden administration for implementing more sanctions against the country in response to its recent missile launches, warning of more challenging and more explicit action if Washington continues to take a "confrontational stance."

An unidentified Foreign Ministry representative praised the North's recent launches of alleged hypersonic missiles as a virtuous exercise of self-defense in a statement released by North Korea's official Korean Central News Agency.

The spokeswoman stated that the additional restrictions demonstrate Washington's hostile desire to "isolate and stifle" the North, despite Washington's repeated requests for Pyongyang to begin negotiations, which has stopped due to disputes over sanctions relief and nuclear disarmament steps.

In reaction to the North's second missile launch this week, the Biden administration imposed penalties on five North Koreans on Wednesday for their participation in obtaining equipment and technology for the North's missile programs. Also, it stated that it would seek fresh UN sanctions.

The Treasury Department's notification came just hours after North Korea claimed leader Kim Jong-un successfully tested a hypersonic missile on Tuesday, which he said would significantly boost the country's nuclear "war deterrent."