The staffer, who attended a briefing by State Department IT officials, says the officials told lawmakers that 60,000 emails were stolen from 10 State Department accounts.

North Korea objects to such exercises saying they are preparations by the US and its South Korean ally for an invasion of it.

The hacking group behind this activity, known in the industry as Midnight Blizzard or APT29, is based in Russia and the UK and US governments have linked it to the country's foreign intelligence service.

A blog post provides the most fulsome explanation yet for a hack that rattled both the cybersecurity industry and China-US relations.

The hacking group, which Microsoft dubbed Storm-0558, forged digital authentication tokens to access webmail accounts running on the firm's Outlook service, reports say.

The hackers, known as APT29 or 'Cozy Bear', intercepted and copied a flyer, embedded it with malicious software, then sent it to dozens of other foreign diplomats working in Kyiv, reports say.

Lazarus has been blamed for a series of brazen thefts targeting cryptocurrency users and organisations.

Many countries around the world that have expressed their support for Ukraine have disclosed similar hits on websites.

Western officials have long warned of cyber threats from rival powers such as Russia or China, but the proliferation of hack-for-hire firms is attracting growing concern.

The hackers 'had access to certain customer data such as names, addresses, email addresses and telephone numbers, but not bank details', a spokesman says.

The firm, dubbed 'Team Jorge' by the team of investigating journalists, was led by former Israeli special forces operative Tal Hanan, who has denied 'any wrongdoing'.

Trickbot's malicious software at one point was counted among one of the internet's most pernicious security threats, capable of stealing financial data, spreading across networks, and dropping ransom software.