One former senior foreign policy official in the Obama administration received messages from someone on LinkedIn offering to fly him to China and connect him with “well paid” opportunities.

A former Danish Foreign Ministry official got LinkedIn messages from someone appearing to be a woman at a Chinese headhunting firm wanting to meet in Beijing. Three middle-aged men showed up instead and said they could help the former official gain “great access to the Chinese system.”

A former Obama White House official and career diplomat was befriended on LinkedIn by a person who claimed to be a research fellow at the California Institute of Technology, with a profile page showing connections to White House aides and ambassadors. No such fellow exists.

Foreign agents are exploiting social media to try to recruit assets, with LinkedIn as a prime hunting ground, Western counterintelligence officials say. Intelligence agencies in the United States, Britain, Germany and France have issued warnings about foreign agents approaching thousands of users on the site. Chinese spies are the most active, officials say.

“We’ve seen China’s intelligence services doing this on a mass scale,” said William R. Evanina, director of the National Counterintelligence and Security Center, a government agency that tracks foreign spying and alerts companies to possible infiltration. “Instead of dispatching spies to the U.S. to recruit a single target, it’s more efficient to sit behind a computer in China and send out friend requests to thousands of targets using fake profiles.”

The use of social media by Chinese government operatives for what American officials and executives call nefarious purposes has drawn heightened scrutiny in recent weeks. Facebook, Twitter and YouTube said they deleted accounts that had spread disinformation about the Hong Kong pro-democracy protests. Twitter alone said it removed nearly 1,000 accounts.

It was the first time Facebook and Twitter had taken down accounts linked to disinformation from China. Many governments have employed similar playbooks to sow disinformation since Russia used the tactic to great effect in 2015 and 2016.