Washington: At least six US states have been targeted by hackers linked to China in the last 10 months as part of a persistent information-gathering operation, cybersecurity company Mandiant has said.

The wide range of state agencies targeted include "health, transportation, labour (including unemployment benefit systems), higher education, agriculture, and court networks and systems," the FBI and US Cybersecurity and Infrastructure Security Agency (CISA) said in a separate, private advisory to state governments obtained by CNN.

For agencies in two states, the hackers broke into networks using a critical software flaw that was revealed in December, according to CNN.

It reported that hundreds of millions of computers around the world ran the vulnerable software, US officials later estimated. For weeks, US officials urged companies to update their software; the White House hosted a meeting in January with tech executives to try to address the root problem of software that is not secure by design.

CNN further reported that within hours of the CISA advisory, the Chinese hackers had begun using the Log4J flaw to break into the two US state agencies, according to Mandiant.

Agencies in four other states were hacked via other means, it added.

As per analysts, Chinese hacking continues to pose a challenge to the Biden administration's efforts to defend government networks. Suspected Chinese hackers compromised at least five US defence and technology firms in an apparent espionage effort, CNN reported in December.

"We assess that China presents the broadest, most active, and persistent cyber espionage threat to US Government and private sector networks," US intelligence agencies said in their annual assessment of global threats released on Tuesday.