The US attorney general said Wednesday that President Donald Trump's administration will intensify its crackdown on what is known as "birth tourism", a day after the Supreme Court issued a ruling affirming birthright citizenship.

The US Supreme Court struck down on Tuesday an executive order issued by Trump at the start of his second term that sought to deny automatic American citizenship to children born to parents residing in the United States illegally or holding temporary visas.

However, acting Attorney General Todd Blanche told reporters on Wednesday that authorities would proceed with measures targeting foreign women who intend to give birth in the United States in order to secure American citizenship for their children.

Blanche said at a press conference that the Justice Department had directed federal prosecutors and law enforcement agencies to take a harder line on "birth tourism", describing the phenomenon as "thriving and set to continue" in light of "yesterday's Supreme Court ruling".

He added: "There are other things the federal government can do in the visa issuance process and the application process to try to reduce or restrict the ability of some people to come here just to have a child who can then become an American citizen."

However, experts consider the phenomenon to be relatively limited compared with more than 250,000 births per year to children of undocumented immigrants or temporary residents in the United States.