The group, nicknamed Salt Typhoon, accessed email systems used by some staffers on the House China committee as well as aides on panels covering foreign affairs, intelligence and the armed services, the report said. It did not identify which specific staffers were targeted.

Reuters could not immediately verify the report. Chinese Embassy spokesman Liu Pengyu condemned what he called “unfounded speculation and accusations”, while the FBI declined to comment.

The White House and the offices of the four committees reportedly targeted in the surveillance sweep did not immediately respond to requests for comment.

The FT cited a person familiar with the campaign as saying it was unclear whether the attackers had accessed lawmakers’ emails in the intrusions, which were detected in December.

US lawmakers and their aides, especially those that oversee America’s sprawling military and intelligence agencies, have long been top targets for cyberespionage and reports of hacks or attempted hacks have surfaced periodically.

In November, the Senate Sergeant at Arms notified multiple congressional offices of a “cyber incident”, where hackers may have accessed communications between the nonpartisan Congressional Budget Office, which provides key financial research data to lawmakers, and some Senate offices.

In 2023, the Washington Post reported that two senior US lawmakers were among the targets of a Vietnam-linked hacking operation.

The Salt Typhoon hackers, in particular, have long rattled the US intelligence community. The spies – alleged to be working for Chinese intelligence – stand accused of gathering data on wide swathes of Americans’ telephone communications and intercepted conversations, including those between prominent US politicians and government officials.

Beijing has repeatedly denied being behind the spying.

Early last year, the US imposed sanctions on alleged hacker Yin Kecheng and cybersecurity company Sichuan Juxinhe Network Technology, accusing both of being involved in Salt Typhoon.