The Justice Department and the FBI said Monday that the online accounts of millions of Americans were caught up in a “vicious” Chinese hacking plot that targeted U.S. officials.

Seven Chinese nationals have been charged with waging a large-scale cyberattack campaign.

They are accused of being linked to a piracy operation that lasted for 14 years.

The US State Department announced a reward of up to $10 million for anyone who provides information about the seven men.

The seven men allegedly sent more than 10,000 “malicious emails, affecting thousands of victims, across multiple continents,” in what the Justice Department called a “prolific global hacking operation” supported by the Chinese government.

“Today’s announcement exposes China’s ongoing and reckless efforts to undermine our country’s cybersecurity and target Americans and our innovations,” FBI Director Christopher Wray said.

He added: “As long as China continues to target the United States and our partners, the FBI will continue to send a clear message that cyber espionage will not be tolerated, and we will tirelessly pursue those who threaten our country's security and prosperity.”

The accusations come after the UK government also accused China of being responsible for “malicious cyber campaigns” targeting the country's electoral commission and politicians. Diplomats at the Chinese embassy in London said they “strongly oppose” these accusations, describing them as “completely fabricated and malicious slander.”

The New Zealand government also said its parliament had been targeted by Chinese-backed hackers, the New Zealand Herald reported.

A spokesperson for the Chinese Embassy in Washington, D.C., said: “Without proper evidence, the countries concerned jumped to an unjustified conclusion” and “made baseless accusations.”

In the indictment outlining the charges against the seven Chinese men, US prosecutors said the hacking led to the confirmed or probable compromise of business accounts, personal emails, online storage and phone call logs.

Emails accused of sending targets often appear to come from prominent news outlets or journalists, and contain hidden tracking links. If someone opened the email sent to them, their information — including their location and IP addresses — would be sent to a server allegedly controlled by the seven defendants.

This information was then used to enable more “direct and sophisticated targeted hacking, such as hacking recipients' home routers and other electronic devices,” US prosecutors said.

In addition to targeting US government officials working in the White House and US State Departments, and in some cases their spouses, they have also reportedly targeted foreign dissidents globally.

In one example cited by the Department of Justice, the men “successfully compromised pro-democracy activists in Hong Kong and their associates based in Hong Kong, the United States, and other foreign locations using similar malware.”

US companies were also hacked, with the men allegedly targeting the defence, ICT, manufacturing, trade, finance, consulting, legal and research industries.

The Justice Department said the companies targeted included defense contractors providing services to the US military and “a leading provider of 5G network equipment.”