Stickers on gadgets warning "warranty void if removed" are deceptive and likely to be illegal in the US, its consumer watchdog has said.
The Federal Trade Commission (FTC) said it had written to six companies to warn them about using misleading language.
Ordering customers to "use specified parts or service providers to keep their warranties intact" was also prohibited, it said.
The companies have 30 days to respond to the FTC.
The commission did not name the companies it had written to, but it said the list included video games companies, smartphone-makers and car manufacturers.
It gave examples of "questionable provisions" in warranty documents, which matched the text in warranty documents issued by Nintendo, Sony and car-maker Hyundai.
The BBC has invited the companies to respond.
"Provisions that tie warranty coverage to the use of particular products or services harm both consumers who pay more for them as well as the small businesses who offer competing products and services," said the FTC's Thomas Pahl.
Stickers that say "warranty void if removed" are often found on gadgets, in many cases covering screws that would let somebody open the product and inspect its internal components.
The stickers can indicate whether a consumer has tried to carry out an "unauthorised repair".
The regulator said such provisions were "generally prohibited by the Magnuson-Moss Warranty Act, a law that governs consumer product warranties".
It has given the six companies 30 days to "revise their practices" and warned that failure to do so could "result in law enforcement action".