The move came as a caravan of thousands of migrants, which Trump has branded an "invasion" of "criminals", makes its way northward through Mexico toward the border.
Later, at a campaign rally in Missouri, Trump offered additional remarks on the topic, saying, "Under this plan, the illegal aliens will no longer get a free pass into the country by lodging meritless claims in seeking asylum".
The Trump administration took a major step on Thursday to restrict asylum claims by migrants, putting forward a regulation that would make individuals ineligible for asylum if they cross the United States southern border illegally.
The interim final rule asserts that the administration can effect these changes under its regulatory authority, but in fact the rule represents an unlawful attempt to change the letter of the statute by regulation.
Currently, U.S. asylum rules do not bar people who enter the country without authorization, and the Immigration and Nationality Act, which governs the U.S. immigration system, specifically allows people who arrive in the United States, whether or not they do so at a designated port of entry, to apply for asylum.
Trump's proclamation puts into practice regulations adopted by immigration officials on Thursday.
White House insiders admit that Donald Trump expects his administration will be sued over its latest immigration policy - but with Brett Kavanaugh on the Supreme Court, he expects to win those suits.
Some officials say the order will help immigration officials process asylum claims in a safer and more efficient manner.
Democrats seek hearings on Trump's ouster of Sessions
They are calling for a "rapid response" to protect Special Counsel Robert Mueller , who is overseeing the investigation. Beverly Hadden, of Cottonwood , said this was part of a nationwide protest coordinated to take place Thursday at 5 p.m.
Every year the U.S. admits more than 1.5 million foreign nationals, with the vast majority deriving from family-based chain migration, whereby newly naturalized citizens can bring an unlimited number of foreign relatives to the U.S. In 2016, the legal and illegal immigrant population reached a record high of 44 million.
What is the new rule? Omar Jadwat, director of the American Civil Liberties Union's Immigrants' Rights Project, said Thursday they were clearly illegal.
The goal appears to be to try to siphon would-be illegal immigrants to those ports of entry and away from jumping the border.
The United States regularly sends Mexican undocumented entrants back across the border to their country, but has had difficulties gaining cooperation to repatriate Hondurans, Salvadorans and Guatemalans.
Trump has also expressed an interest in ending birthright citizenship to those born to non-citizens and for ending the the so-called DACA protections of those who've been in the usa for years but were brought into the country illegally while children.
He called a current caravan, which is still hundreds of miles from the USA border and dwindling in numbers, an "invasion" and said it would bring hardened criminals to United States streets.
It's unclear how many people en route to the USA will even make it to the border. Several smaller groups were trailing hundreds of kilometres to the south; officials estimated about 7,000 in all were in the country in the caravans.