WASHINGTON (Reuters) -The U.S. Food and Drug Administration will send out around 100 cease-and-desist enforcement notices and thousands of letters warning pharmaceutical companies that direct-to-consumer ads must comply with regulations that are already on the books, senior administration officials said on Tuesday.

The FDA plans to enforce regulations stipulating that drug ads not create a misleading impression about the products and appropriately disclose side effects, the officials said.

"There are ads that are clearly crossing the line with respect to the regulation, making any potential future legal action, I think, pretty clear cut," one of the officials said.

U.S. President Donald Trump also signed a presidential memorandum on Tuesday afternoon, the White House said, calling on his administration to step up enforcement of direct-to-consumer pharmaceutical ads in order to ensure transparency and accuracy.

"The Secretary of Health and Human Services shall therefore take appropriate action to ensure transparency and accuracy in direct-to-consumer prescription drug advertising," the memorandum said.

The presidential action and FDA letters coincide with the release of the report on U.S. children's health by U.S. Health Secretary Robert F. Kennedy Jr.'s Make America Healthy Again Commission.

The officials said that enforcement of drug ads has been increasingly lax, adding that last year, the FDA did not send out any enforcement letters about drug advertising.

They did not disclose which companies had been sent the enforcement or warning letters. They said they were not only concerned about ads from drugmakers, but also ads from online pharmacies "that are not following the same rules that many pharmaceutical companies follow."

The government is also examining the role social media influencers play in advertising drugs and plans to close a loophole that allows companies to refer patients to a website for information on side effects, the officials said.

Administration officials said there are no additional presidential actions planned on direct-to-consumer drug ads, calling the memorandum "the strongest, boldest action that we can take on making sure that patients have adequate safety information."

In his first term, Trump attempted to push through a rule that would have forced pharmaceutical companies to include the wholesale prices of their drugs in television advertising.

U.S. courts sided with drugmakers Merck, Eli Lilly and Amgen and struck down the rule.

U.S. pharmaceutical lobby group PhRMA said its member companies "are committed to responsible advertising, and we look forward to learning more details about the policy changes announced today."