(Reuters) – The Chinese government has rejected claims that its spies are penetrating Western infrastructure, calling the joint warning issued by the United States and its allies a “collective disinformation campaign.”

Chinese foreign ministry spokesperson Mao Ning told reporters that alerts issued by the U.S., Britain, Canada, Australia and New Zealand were intended to promote their intelligence alliance, known as the Five Eyes – and that it was Washington that was guilty of hacking,

“The United States is the empire of hacking,” Mao said.

The reaction follows a series of warnings issued by Five Eyes countries – and major U.S. tech firm Microsoft Corp (NASDAQ:MSFT) – about the activities of a Chinese hacking group known as Volt Typhoon.

Although Chinese spies have long been active online against the United States and its allies, Volt Typhoon has raised particular concerns because of its focus on critical infrastructure, including communications links that tie the United States to the Pacific, analysts say.

The group’s focus on stealthiness is also drawing attention.

Cybersecurity company Secureworks, which said it has responded to at least three Volt Typhoon hacks, described the group as working consistently to cover its tracks.

The company also backed Western assessments of the group’s origins, saying that the hacker group, which it nicknamed “Bronze Silhouette,” likely operates on behalf of Beijing.

Secureworks – an arm of Dell Technologies (NYSE:DELL)’ – said that Chinese spies were upping their game in response to “likely increased pressure from (Chinese) leadership to avoid public scrutiny of its cyberespionage activity.”