Donald Trump, the US president, on Friday invoked extraordinary national security powers to deny asylum to migrants who enter the country illegally, tightening the border as caravans of Central Americans slowly approach the United States.

Mr Trump is using the same powers he used to push through a version of the travel ban that was upheld by the Supreme Court. The proclamation puts into place regulations adopted Thursday that circumvent laws stating that anyone is eligible for asylum no matter how he or she enters the country.

"We need people in our country but they have to come in legally and they have to have merit," Mr Trump said Friday as he prepared to depart for Paris.

Administration officials say the measures are in effect for at least three months, but could be extended. They go into effect Saturday and don't affect people who are already in the country.

The changes are meant to funnel asylum seekers through official border crossings for speedy rulings, officials said, instead of having them try to circumvent such crossings on the nearly 2,000-mile (3,200-kilometer) border.