Melania Trump - until now only a minor presence on the campaign trail - found herself caught in an embarrassing plagiarism controversy on Tuesday after a prime-time defense of her husband that appeared to be lifted in part from a speech given by first lady Michelle Obama.

No sooner had the poised 46-year-old former model delivered her keynote address to cheering delegates at the Republican National Convention than the unmistakable similarities to a passage from Obama's speech to the 2008 Democratic convention came to light.

In both passages, the women are introducing themselves to the US public by speaking of the values that have shaped their lives.

"My parents impressed on me the values: that you work hard for what you want in life. That your word is your bond and you do what you say and keep your promise. That you treat people with respect. They taught me to show the values and morals in my daily life. That is the lesson that I continue to pass along to our son," Melania Trump said.

"And we need to pass those lessons on to the many generations to follow. Because we want our children in this nation to know that the only limit to your achievements is the strength of your dreams and your willingness to work for them."

In her speech, Michelle Obama said:

"And Barack and I were raised with so many of the same values: that you work hard for what you want in life; that your word is your bond and you do what you say you're going to do; that you treat people with dignity and respect, even if you don't know them, and even if you don't agree with them.

"And Barack and I set out to build lives guided by these values, and pass them on to the next generation. Because we want our children - and all children in this nation - to know that the only limit to the height of your achievements is the reach of your dreams and your willingness to work for them."

Trump's senior communications adviser, Jason Miller, issued a statement that sidestepped the plagiarism question while not denying it.

"In writing her beautiful speech, Melania's team of writers took notes on her life's inspirations, and in some instances included fragments that reflected her own thinking," he said. "Melania's immigrant experience and love for America shone through in her speech, which made it such a success."