A British man has been indicted in the United States on charges of attempting to pass “sensitive American technology” to China.

John Miller, 63, was indicted by U.S. federal juries on Friday along with Chinese national Cui Guanghai for allegedly trying to export a device used for encryption and decryption to Beijing, according to a statement from the United States Attorney’s Office for the Central District of California.

The two men, who were arrested in Serbia, discussed ways to smuggle the device to China via Hong Kong in a blender, according to the statement.

The pair “solicited the procurement of U.S. defense articles, including missiles, air defense radar, drones and cryptographic devices with associated crypto ignition keys for unlawful export from the United States to the People’s Republic of China,” the U.S. Attorney’s Office said.

The case comes amid heightened tensions between the U.S. and China, as the two engage in an escalating trade war. On Sunday, Beijing warned Washington to not “play with fire” after U.S. defense chief Pete Hegseth said China could be gearing up to invade Taiwan.

Both men are also accused of attempting to “harass” a Chinese-American artist and known critic of Beijing by trying to install a tracking device on the victim’s car and slashing his tires. The U.S. is seeking to extradite the pair from Serbia.

The two men could face up to 20 years in prison under the U.S. Arms Export Control Act if found guilty, and 10 years for smuggling.

U.S. Deputy Attorney General Todd Blanche called the plot a “blatant assault” on the country’s national security, adding the American government would not “allow hostile nations to infiltrate or exploit our defense systems.”

Miller, who lives in Kent in the U.K. and describes himself as a recruitment specialist, reportedly referred to Chinese President Xi Jinping as “the boss” in intercepted phone calls. He was caught in a sting operation after discussing his plans with FBI agents posing as arms dealers, according to media reports.

Britain’s Foreign Office said it was providing consular assistance to Miller following his arrest in Serbia on April 24.