The guidance issued by the FBI and the Cybersecurity and Infrastructure Security Agency is intended to help root out the hackers and prevent similar cyberespionage in the future. Officials who briefed reporters on the recommendations said the U.S. still doesn’t know the true scope of China’s attack or the extent to which Chinese hackers still have access to U.S. networks.

In one sign of the global reach of China’s hacking efforts, the government’s warning was issued jointly with security agencies in New Zealand, Australia and Canada, members of the Five Eyes intelligence alliance, which also includes the U.S. and Britain.

Dubbed Salt Typhoon by analysts, the wide-ranging cyberespionage campaign emerged earlier this year after hackers sought to penetrate the networks of multiple telecommunications companies.

The hackers used their access to telecom networks to target the metadata of a large number of customers, including information on the dates, times and recipients of calls and texts.

The hackers succeeded in retrieving the actual audio files of calls and content from texts from a much smaller number of victims. The FBI has contacted victims in this group, many of whom work in government or politics, but officials said it is up to telecom companies to notify customers included in the first, larger group.

Despite months of investigation, the true scale of China’s operation, including the total number of victims or whether the hackers still have some access to information, is currently unknown.

The FBI has said some of the information targeted by the hackers relates to U.S. law enforcement investigations and court orders, suggesting the hackers may have been trying to access programs subject to the Foreign Intelligence Surveillance Act, or FISA. The law grants American spy agencies sweeping powers to surveil the communications of people suspected of being agents of a foreign power.

But on Tuesday, officials said they think the hackers were more broadly motivated, hoping to burrow deeply into the nation’s telecommunications systems to gain wide access to Americans’ information.

The suggestions for telecom companies released Tuesday are largely technical in nature, urging encryption, centralization and consistent monitoring to deter cyber intrusions. If implemented, the security precautions could help disrupt the Salt Typhoon operation and make it harder for China or any other nation to mount a similar attack in the future, said Jeff Greene, CISA’s executive assistant director for cybersecurity and one of the officials who briefed reporters Tuesday.

“We don’t have any illusion that once we kick off these actors they’re not going to come back,” Greene said.

Several recent high-profile hacking incidents have been linked to China and what officials say is Beijing’s effort to steal technical and government secrets while also gaining access to critical infrastructure such as the electrical grid.

In September, the FBI announced that it had disrupted a vast Chinese hacking operation that involved the installation of malicious software on more than 200,000 consumer devices, including cameras, video recorders and home and office routers. The devices were then used to create a massive network of infected computers, or botnet, that could then be used to carry out other cyber crimes.

In October, officials said hackers linked to China targeted the phones of then-presidential candidate Donald Trump and his running mate, Sen. JD Vance, along with people associated with Democratic candidate Vice President Kamala Harris.

China has rejected accusations from U.S. officials that it engages in cyberespionage directed against Americans.

On Tuesday, a spokesperson for China’s embassy in Washington called the U.S. allegations “disinformation.”

China’s government “firmly opposes and combats all kinds of cyber attacks,” spokesperson Liu Pengyu wrote in a statement emailed to The Associated Press. “The US needs to stop its own cyberattacks against other countries and refrain from using cyber security to smear and slander China.”