The top Republican tells WVLK radio in Kentucky, "Well you obviously can not do that".
However critics have fired back, saying the move is unconstitutional and Mr Trump has no authority to issue a planned executive order.
Ryan's comments Tuesday offered a rare challenge to the president - from his own party.
Birthright citizenship is the law of at least 30 countries, including numerous U.S.'s neighbors in North and South America. "But the 14th amendment of the constitution is very clear". "We're not catching, we're not releasing", he said.
The President didn't provide any details of his plan, but said that "it's in the process".
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But Trump said he has talked to his legal counsel and was advised he could enact the change on his own, contrary to the view of many constitutional experts.
"We all know what the 14 Amendment says".
By extending citizenship to those born in the US, the amendment nullified an 1857 Supreme Court decision (Dred Scott v. Sandford), which ruled that those descended from slaves could not be citizens.
But conservatives say that right should not apply to everyone, including immigrants in the country illegally or those with temporary legal status.
With the midterms days away, Trump has said he wants to unilaterally end the constitutional right to citizenship for babies born in the States to non-citizens.
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Trump's remarks mark another escalation in his hardline stance on immigration ahead of United States midterm elections on November 6 that could see the Democrats regain some degree of power. "Guess what? You don't", Trump said in an interview with Axious.
White House lawyers expect to work with the Justice Department to develop a legal justification for the action. "And so this was essentially putting out a welcome mat to immigrants by ensuring that their children born here would be citizens". Lindsey Graham, R-S.C., announced in a series of tweets that he will introduce legislation to end birthright citizenship, the Hill reports.
Because it is enshrined in the Constitution, there is a question whether Trump can undo it with an executive order.
This was the answer U.S President Donald Trump gave in regard to ending the right to U.S citizenship to children born on USA soil to parents who are not citizens.
Trump's comments come as he tries to focus the debate on immigration and a caravan of Central American migrants heading to the U.S. border as the country a week before the midterm elections reels from the massacre of 11 worshipers at a Pittsburgh synagogue and pipe-bomb mailings to a number of prominent Democrats.
Trump will be in full campaign mode in final week before midterms
But according to the Cook Report, which analyzes individual House races, the Democrats only have a net 16-seat polling advantage. The Florida Democratic Party said RSVP information for Friday's Miami rally will be available in the coming days.