The US arm of China's largest bank ICBC said it was hit by a ransomware attack, forcing clients to reroute trades.

Ransomware attacks typically access vulnerable computer systems and encrypt or steal data, before sending a ransom note demanding payment in exchange for decrypting the data or not releasing it publicly.

The Industrial and Commercial Bank of China Financial Services (ICBC FS) said Thursday it "experienced a ransomware attack that resulted in disruption to certain (financial services) systems."

"Immediately upon discovering the incident, ICBC FS disconnected and isolated impacted systems to contain the incident," the New York-based bank said, adding that it was investigating the attack and working on recovery.

ICBC FS said it had successfully cleared US Treasury trades executed Wednesday and repurchasing (repo) financing trades Thursday.

Slack demand for $24 billion in 30-year US Treasury bonds that were auctioned Thursday came as a surprise to some analysts.

This sale "attracted very poor demand, one of the weakest I can remember," Karl Haeling of the bank LBBW told AFP.

Richard Flax, chief investment officer at Moneyfarm, said that some commentators believed the cyber attack "rather than weak demand, was behind the relatively poor US government bond auction."

However US Treasury Secretary Janet Yellen downplayed the attack's effects, stating Friday that "we've not seen an impact on the Treasury market."

- 'Work together' -

Yellen, who spoke during a press conference on the second day of bilateral meetings with her Chinese counterpart He Lifeng in San Francisco, said the issue was an example of why China and the United Sates "need close communications."

"It's critical to be able to pick up the phone and know that you will have a good response on the other end, and that we can trust one another," she said, as Washington works to reconnect with Beijing despite recent economic and political tensions.

Bloomberg reported that some trades handled by ICBC FS on Thursday were transported across Manhattan on a USB stick as messengers manually relayed required settlement details.

China's foreign ministry said Friday that "the business systems and office systems of the head office of ICBC and other domestic and foreign branches and subsidiaries within the group are normal."

"As far as we know, ICBC has paid close attention to this matter, and has done a good job in emergency handling and supervision and communication, striving to minimize the impact of risks and losses," foreign ministry spokesman Wang Wenbin said at a regular news briefing.

"At present, the business systems and office systems of the head office of ICBC and other domestic and foreign branches and subsidiaries within the group are normal."

US media reported that the hack was executed using software created by LockBit, the Russian-speaking hacking group known for scrambling files on a host's computer and flashing up messages demanding cryptocurrency payment to resolve the issue.

"The infiltration of a financial giant like ICBC serves as a reminder that no target is deemed off-limits in the eyes of these groups," said Amelia Buck, cybersecurity expert at Menlo Security.

US aircraft manufacturer Boeing was hit with an attack from LockBit last week.

Last year, LockBit was "the most deployed ransomware variant across the world and continues to be prolific in 2023," according to the US Cybersecurity and Infrastructure Security Agency.

The US Justice Department said in May that LockBit ransomware had been used in more than 1,400 attacks globally.

LockBit has targeted critical infrastructure and large industrial groups, with ransom demands ranging from EUR5 million to EUR70 million.

The group attacked Britain's Royal Mail in early January and a Canadian children's hospital in December.