China's state security agency on Wednesday warned students with access to sensitive information against falling for "handsome men" or "beautiful women" that might entice them to spy for foreign powers.

Beijing's Ministry of State Security (MSS) has promoted claims that foreign spies are working to lure loyal Chinese to betray their country -- often in lurid and unusual ways -- since opening a WeChat account last year.

It has warned that foreign spies "have countless disguises, and can even change their gender" and called on citizens to "build 1.4 billion lines of defence" against threats to the country.

And it accused on Wednesday foreign intelligence agencies of deploying "romance traps" to lure Chinese students.

It said foreign spies were using job ads and even online dating to "lure and coerce" young students, especially those with access to "sensitive scientific research data", to hand over confidential information.

"They may even disguise themselves as 'handsome men' or 'beautiful women'... and drag young students into 'romance trap'," it warned.

The MSS did not name which countries were behind the alleged scheme.

But it warned that spies might be disguising themselves as university scholars, scientific researchers or consultants, luring students short of cash in what it called "targeted infiltration".

In another lurid tale this month, the MSS warned the public to beware "wolves in sheep's clothing" -- foreign agents pretend to be "good Samaritans".

And in June, it accused Britain's MI6 intelligence agency of recruiting a couple who worked for the central government to spy for the UK.

Under Xi Jinping, China's most powerful and authoritarian leader in decades, Beijing has stepped up dark warnings that foreign powers are seeking to clip the country's rise.

China and Western powers have long traded accusations of spying but only recently started to disclose details of alleged individual cases.

In May, investigators searched the Brussels office of a German MEP suspected of spying for China.

And on Tuesday, a former official who worked as an aide to the governor of New York state was arrested and charged with working as an agent of China in return for millions of dollars.