Both service personnel are based in California, but the cases have been brought separately and federal officials have declined to be drawn over whether there are links between them.

The two men pleaded not guilty at courts in San Diego and Los Angeles and were remanded in custody until their detention hearings next week.

The US has raised concerns for a number of years about the spying threat posed by Beijing, bringing criminal cases against Chinese agents who have stolen sensitive government and commercial information, including through illegal hacking.

The latest prosecutions come as a Massachusetts Air National Guardsman is facing trial for leaking highly classified military documents.

Jack Teixeira, 21, is accused of releasing secret information about the Ukraine war and other sensitive topics in a social media chat room popular with online gamers.

The authorities say the cases linked to China underline Beijing's blatant attempts to gather details on US military operations.

US attorney Randy Grossman for the southern district of California said: "Through the alleged crimes committed by these defendants, sensitive military information ended up in the hands of the People's Republic of China."

He added the charges demonstrate the Chinese government's "determination to obtain information that is critical to our national defence by any means, so it could be used to their advantage".

Jinchao Wei, a 22-year-old sailor serving on the San Diego-based USS Essex, was arrested on Wednesday while boarding the ship.

He is accused of passing detailed information on the weapons systems and aircraft aboard the vessel and other amphibious assault ships.

Prosecutors said Wei, who was born in China, was approached by a Chinese intelligence officer in February 2022 while he was applying to become a naturalised US citizen.

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The indictment alleges Wei provided photographs and videos of navy ships, as well as many as 50 manuals containing technical and mechanical data about vessels along with details about the number and training of US marines during an upcoming exercise.

Wei sent sensitive US military information over the course of a year and was even congratulated by the Chinese officer on becoming a US citizen, Mr Grossman said.

He added Wei "chose to turn his back on his newly adopted country" for greed.

Assistant US Attorney Fred Sheppard told the judge Wei had passed information to Chinese intelligence as recently as this week.

Mr Sheppard said Wei made up to $15,000 (£11,800) in the past year from the arrangement.

If convicted, he could face up to life in prison.

The Justice Department also charged Wenheng Zhao, 26, serving at a naval base north of San Diego, with conspiring to take nearly $15,000 (£11,800) in bribes from a Chinese agent in exchange for US naval exercise plans, operational orders and photos and videos of electrical systems at navy facilities between August 2021 until at least this May.

The information included operational plans for a large-scale US military exercise in the Indo-Pacific region, which detailed the location and timing of naval force movements.

The indictment alleges Zhao photographed electrical diagrams and blueprints for a radar system stationed on a US military base in Okinawa, Japan.

If convicted, Zhao could face a maximum sentence of 20 years in federal prison.

Speaking at the Pentagon, Brigadier General Pat Ryder said: "I think we have clear policies and procedures in place when it comes to safeguarding and protecting sensitive information. And so if those rules are violated, appropriate action will be taken."

Mr Grossman said: "China is unrivalled in its audacity and the range of its maligned efforts to subvert our laws."

He warned the US would use "every tool in our arsenal to counter the threat and to deter China and those who have violated the rule of law and threaten our national security".