The White House should investigate whether presidential adviser Kellyanne Conway violated government ethics rules when she made a public endorsement of Ivanka Trump products, and consider disciplinary action, the Office of Government Ethics said in a letter on Tuesday.

Conway's comments last week after retailer Nordstrom announced it was dropping the branded line of Trump's daughter Ivanka prompted criticism from Republican and Democratic lawmakers amid concern she may have violated ethics rules.

"There is strong reason to believe that Ms. Conway has violated the Standards of Conduct and that disciplinary action is warranted," the letter read.

The White House did not immediately reply to a request for comment.

Federal ethics rules prohibit executive branch employees from using their positions to endorse products or for the private gain of friends. The law does not apply to the president.

"Go buy Ivanka's stuff ... I'm going to go get some myself today," Conway told Fox News in an interview from the White House. "I'm going to give a free commercial here: Go buy it today, everybody."

Conway made the comments on Thursday, a day after Trump himself attacked Nordstrom for dropping his daughter's brand.

Nordstrom said it had made the decision because sales had steadily declined, especially in the last half of 2016, to where carrying the line "didn't make good business sense."

A congressional committee sought a review from the OGE into whether Conway had violated ethics in her televised remarks.

"I note that the OGE's regulation on misuse of position offers as an example the hypothetical case of a presidential appointee appearing in a television commercial to promote a product," OGE Director Walter Shaub wrote in his letter. "Ms. Conway's actions track that example almost exactly."

"Therefore I recommend that the White House investigate Ms. Conway's actions and consider taking disciplinary action against her," he wrote.

Shaub noted that White House spokesman Sean Spicer said on Thursday that Conway had been "counseled," but said OGE had not been informed of any disciplinary or corrective action.

Shaub asked White House lawyers to respond by Feb. 28.

A top White House aide defended Conway on Sunday. Trump policy adviser Stephen Miller told ABC's "This Week" that she was making a "light-hearted, flippant" comment when she urged Americans to buy Ivanka Trump's products.