US President Donald Trump is vowing to sign an executive order that would seek to end the 150-year-old right to American citizenship for children born in the country to noncitizens.

"We're the only country in the world where a person comes in and has a baby, and the baby is essentially a citizen of the United States for 85 years with all of those benefits," Trump said during an interview with Axios scheduled to air on HBO this weekend. "It's ridiculous. It's ridiculous. And it has to end."

More than 30 other countries, however, including Canada and Mexico, grant citizenship to children born within their borders.

"It was always told to me that you needed a constitutional amendment. Guess what? You don't," Trump said. "You can definitely do it with an act of Congress. But now they're saying I can do it just with an executive order."

The Supreme Court has upheld the rule for legal permanent residents but has never decided a case involving an illegal immigrant or a short-term visitor to the US.

Trump saying he could change the Constitution by executive order was opposed by Democrats, many legal scholars and a top Republican.

"Well you obviously cannot do that," House Speaker Paul D. Ryan, Republican of Wisconsin, said in a radio interview on Tuesday. "You cannot end birthright citizenship with an executive order."

"He's doing something that's going to upset a lot of people, but ultimately this will be decided by the courts," said Saikrishna Prakash, a constitutional expert and University of Virginia Law School professor.

A Chinese woman who recently had a baby in Irvine, California, who asked to remain anonymous, told China Daily: "I don't think he (Trump) can alter the Constitution, and the rights of the children born in the country are not going to be changed easily. So it (the announcement) will not affect our plan.

The first sentence of the 14th Amendment passed after the Civil War establishes the principle of "birthright citizenship": "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside."

The Supreme Court has upheld the rule for legal permanent residents but has never decided a citizenship case involving an illegal immigrant or a short-term US visitor.

The idea behind the amendment was to grant citizenship to recently freed slaves.