One of Donald Trump's most senior advisers "violated" federal law when she endorsed his daughter's product line on live TV, according to two former senior US government officials.

"Go buy Ivanka's stuff, is what I would say," Kellyanne Conway told interviewers on the Fox & Friends TV program. "It's a wonderful line."

Her comments followed an angry tweet from the US President about the decision by Nordstrom, a chain of department stores, to drop Ms Trump's products.

Ms Conway appeared to be standing inside the White House press briefing room as she made her remarks, which were immediately picked up on by Chris Lu, the former US Deputy Secretary of Labour.

Highlighting a section of statute in a screenshot, Mr Lu said: "This is the federal ethics law that @KellyannePolls just violated". He tagged Congressman Jason Chaffetz, chairman of the Congress oversight committee.

Norm Eisen, who served as Special Counsel for Ethics and Government Reform in the White House, before going on to become the US ambassador to the Czech Republic, agreed.

Quoting Mr Lu, he said: "Exactly right!"

Both men served under President Obama.

The law, 5 CFR 2635.702, which Mr Lu cited, states: "An employee shall not use his public office for his own private gain, for the endorsement of any product, service or enterprise, or for the private gain of friends, relatives, or persons with whom the employee is affiliated in a nongovernmental capacity".

Ms Conway twice praised Ms Trump as a "very successful businesswoman" but said: "Obviously she's stepped away from it now".