The attack is said to have hit more than 300,000 computers in 150 nations, causing billions of dollars of damage.
Thomas Bossert, an aide to US President Donald Trump, made the accusation in the Wall Street Journal newspaper.
It is the first time the US has blamed the country officially for the worm.
Mr Bossert, who advises the president on homeland security, said the allegation was “based on evidence”.
He did not produce any evidence in the article, but said US findings concurred with judgments from other governments and private companies.
The United Kingdom government said in November that it was “all but certain” that North Korea carried out the attack.
In May, Windows computers hit by the cyber-attack had their contents locked, with users asked to a pay a ransom to have their data restored. EU police body Europol called the scale of the attack “unprecedented”.
In the piece, Mr Bossert said North Korea must be held “accountable” and said the US would continue to use a “maximum pressure strategy” to hinder the regime’s ability to mount cyber-attacks.
He did not specify what action, if any, the US government planned to take in response to their findings.
The country is already facing major economic sanctions after being re-designated a state-sponsor of terrorism last month amid tension over the North Korean nuclear programme and missile tests.
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“North Korea has acted especially badly, largely unchecked, for more than a decade, and its malicious behaviour is growing more egregious. WannaCry was indiscriminately reckless,” Mr Bossert wrote.
“As we make the internet safer, we will continue to hold accountable those who harm or threaten us, whether they act alone or on behalf of criminal organizations or hostile nations,” he went on.
“The tool kits of totalitarian regimes are too threatening to ignore.”
The White House is expected to give an official statement blaming Pyongyang on Tuesday.
‘Unprecedented attack’
In the UK, the National Health Service (NHS) was hit particularly hard by the cyber-attack, with 48 affected health trusts forced to turn many patients away for appointments and even surgeries.
It spread across the world, with Russia reportedly being badly hit, causing problems to the country’s postal service.
The entertainment company had its films leaked and details of corporate finances and private emails released online.
The North Koreans hit out at former president Barack Obama over the claim, but has not yet responded to the White House accusations about the WannaCry hack.
In October it said rumours from a UK government minister that they were behind the 2017 attack was “groundless speculation”, and a “wicked attempt” to tighten international sanctions on the country.
