Secretary of the Navy Richard Spencer warned in an internal memo in October that " attempts to steal critical information are increasing in both severity and sophistication," The Wall Street Journal reported Friday.

The secretary has launched a review of the service branch's cybersecurity weaknesses, as Chinese hackers target contractors and subcontractors to steal military secrets.

China allegedly targets US military service branches, commercial entities, and even critical national security infrastructure, and these attacks appear to be on the rise.

US Navy defense contractors and subcontractors have reportedly suffered "more than a handful" of disconcerting security breaches at the hands of Chinese hackers over the past year and a half.

"Attacks on our networks are not new, but attempts to steal critical information are increasing in both severity and sophistication," Secretary of the Navy Richard Spencer said in an internal memo in October, The Wall Street Journal, which reviewed the memo, reported Friday.

"We must act decisively to fully understand both the nature of these attacks and how to prevent further loss of vital military information," he added.

Although the secretary did not mention China specifically, evidence indicates that Beijing is responsible for what is considered a debilitating cyber campaign against the US.

Earlier this year, Chinese government hackers stole important data on US Navy undersea-warfare programs from an unidentified contractor. Among the stolen information were plans for a new supersonic anti-ship missile, The Washington Post, citing US officials, reported in June.

China has been striving to boost its naval warfighting capabilities, and there is evidence that it is relying on stolen technology to do so.

And it's not just the US Navy. Adm. Philip Davidson, the head of US Indo-Pacific Command, told the Senate Armed Services Committee in April that Beijing is "stealing technology in just about every domain and trying to use it to their advantage."

China is believed to have been behind multiple cybersecurity breaches that facilitated the theft of significant amounts of data on the F-22 and the F-35, among other aircraft. That information is suspected to have played a role in the development of China's new fifth-generation stealth fighters.

Beijing denies that it engages in any form of cyberespionage.

A senior US intelligence official warned Tuesday that concerning Chinese cyberactivity in the US is clearly on the rise, and there is evidence that China is targeting critical infrastructure to lay the groundwork for disruptive attacks, Reuters reported.

And US officials say Chinese state hackers are responsible for a data breach at Marriott that affected 500 million customers, according to recent reports. The Trump administration has repeatedly criticized Beijing for the alleged theft of US intellectual property that's worth several hundred billion dollars a year, one of several sticking points in the ongoing trade spat.

The breaching of US defense contractor networks is particularly problematic as China modernizes its force, building a military capable to challenge the US.

"It's extremely hard for the Defense Department to secure its own systems," Tom Bossert, the former homeland security adviser in the Trump administration, told The Journal. "It’s a matter of trust and hope to secure the systems of their contractors and subcontractors."

Contractors and subcontractors across the entire military lack the desired cybersecurity capabilities and regularly suffer serious breaches, an intelligence official said.

The most active Chinese hackers are reportedly a group known as Temp.Periscope or Leviathan, which is focused on maritime interests but also hits other targets.

One defense official told The Journal that China was targeting America's "weak underbelly," calling cybersecurity breaches "an asymmetric way to engage the United States without ever having to fire a round."