Seven Chinese citizens were arrested on charges of illegally entering Guam around the time the U.S. Missile Defense Agency conducted a key missile interception test using a new radar, the island's Customs and Quarantine Agency has said.

The agency said at least four of the people arrested on Dec. 10-11 were found "in the vicinity of a military installation". Guam has many such installations, including Andersen Air Force Base, where the Dec. 10 missile test was conducted.

"Conducting espionage against U.S. military facilities, especially those with missile launch capabilities, could provide the PRC with potentially valuable intelligence," the Institute for the Study of War said in a report on Friday.

Guamanian authorities said that all of the Chinese nationals arrived on the same boat from Saipan, and that the investigation was ongoing.

China's Ministry of Foreign Affairs did not immediately respond to a request for comment.

The United States plans to create an air and missile defense network at 16 sites around the island to deter any missile attack on Guam by making it too complex and resource-intensive to carry out.

The plan aims to integrate the most advanced U.S. missile defense systems and radars, and could cost as much as $10 billion over the next decade.

The Dec. 10 test was successful, and the Missile Defense Agency says up to two interception tests could be conducted each year.

Reuters

A U.S. Navy MH-60S Knighthawk, U.S. Air Force RQ-4 Global Hawk, Navy MQ-4C Triton, Air Force B-52 Stratofortresses, and KC-135 Stratotankers perform an 'Elephant Walk' at Andersen Air Force Base, Guam April 13, 2020. U.S. Air Force/Senior Airman Michael S. Murphy/Handout via REUTERS/File Photo

Seven Chinese nationals tried to illegally enter Guam as U.S. tested missile, authorities say