Chinese state media sent a clear warning message this week: as the number of young people studying abroad rises, so, too, do risks to the country's national security.

On Wednesday night, state broadcaster CCTV reported that a young man who had studied abroad in an unnamed country had been caught in 2013 taking photos of a military complex in the eastern Chinese city of Ningbo. Chinese students who have returned from studying abroad are typically called haigui, or "sea turtles," in Mandarin.

The broadcaster said the man, named Chen Wei, had assisted a foreign contact in helping track the movement of Chinese coast guard boats and other vessels patrolling a disputed island chain in the East China Sea.

The islands are controlled by Japan, which calls them the Senkaku Islands. But they are also claimed by China, which has its own name for them, the Diaoyu Islands. At the time the incident occurred three years ago, tensions ran particularly high between China and Japan, with the former dispatching ships and planes into the region.

"The year 2013 was the most sensitive time for the situation around the Diaoyu Islands," CCTV said. "If this information was grasped by another party, should there be a minor incident that were to spark a war, our nation's security would be seriously threatened.”

Mr. Chen received “200,000 units of a foreign currency,” according to CCTV which did not name the currency. He was sentenced to seven years' imprisonment; it wasn't clear when. It wasn't possible to reach Mr. Chen for comment.

"Right now, the number of people studying and working abroad is rising. When people leave their homes, they pay a lot of attention to their personal and property security. Actually, people should be even more mindful of another kind of security: national security," CCTV said.

In this photo provided by Japan Coast Guard, an armed Chinese coast guard ship sails in the water near islands, known as the Senkaku in Japanese and the Diaoyu in Chinese, Tuesday, Dec. 22, 2015. Photo: Associated Press

As incomes have risen, the number of Chinese students choosing to go abroad has likewise vaulted. Between 2008 and 2015, the number of Chinese students in U.S. schools more than tripled to 304,040, according to data from the Institute of International Education.

China isn't alone in worrying that students abroad present rich targets for foreign intelligence agencies. In 2014, the FBI released a 30-minute video cautioning American students in China against the lure of attractive female handlers and easy cash. The film was based on the real-life case of Glenn Shriver, a U.S. citizen found to have taken some $70,000 from people affiliated with the Chinese government. In 2011, he pleaded guilty to conspiring to provide national defense information to unauthorized individuals.

It wasn’t clear why Mr. Chen’s case was only being disclosed now, although the report comes as Beijing is trying to ratchet up awareness about the need to be vigilant against spies of all stripes.

Earlier this week, state media also publicized the case of a former computer technician sentenced to death after he sold sensitive materials to foreign spies between 2002 and 2011, the year he was detained. Reports didn't state which country received the materials or what year the man was sentenced.

A poster warning against foreign spies is displayed in an alleyway in Beijing, China, Wednesday, April 20, 2016. Photo: Associated Press

Such cautionary tales have been circulated in the wake of last Friday's first-ever National Security Education Day, which in Beijing was marked by the hanging of posters warning locals about the risks of dating foreigners. Titled "Dangerous Love," the cartoon-style posters featured a charming red-haired foreigner who claims he's a visiting scholar and in that role manages to entice his Chinese girlfriend to hand over sundry government documents.

The bespectacled foreigner is seen giving the ostensible object of his affection flowers and praising her beauty and gentleness. "These materials will be very helpful for me as I write academic articles," he tells her. She ends up sitting handcuffed, crying, in a police station.

--Te-Ping Chen. Follow her on Twitter @tepingchen.