If you follow celebrities on Instagram, you probably find it hard to tell which posts are ads. That could soon change.

The Federal Trade Commission has sent over 90 reminders to what it calls "influencers" -- celebrities, athletes, models and the like -- asking them to clearly state at the beginning of each post if they were paid to endorse a product or service, according to a statement released Wednesday.

The FTC said any "material connection" should be made clear in the first three lines of a photo's caption, so all mobile users can see it without needing to click "more." The FTC added that a simple #sp (sponsored post) or "thanks [Brand]" aren't sufficient disclosures.

Sponsored posts on Instagram are big business. Kim Kardashian, who has nearly 98.4 million followers, can rake in around $500,000 for a single endorsement "campaign," according to Us Weekly. Anyone with 3 million to 7 million followers can expect a cool $75,000 for an endorsement on Instagram, reported Esquire.

FTC did not reveal the list of recipients, although it said the letters were sent partly in response to petitions and requests from watchdog group Public Citizen.

The US isn't the only country with its eye on sponsored social-media posts. The Australian Association of National Advertisers last month added new guidelines stipulating that influencers must clearly identify all sponsored content.

Instagram declined to comment.

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