“Why are we having all these people from shithole countries come here?” Mr Trump told lawmakers on Thursday, according to the Washington Post.
The remark was reportedly in reference to people from Haiti, El Salvador and African countries.
The White House did not deny the comment, which has been confirmed by other US media.
“Certain Washington politicians choose to fight for foreign countries, but President Trump will always fight for the American people,” a statement from White House spokesman Raj Shah said.
It continued: “Like other countries that have merit-based immigration, President Trump is fighting for permanent solutions that make our country stronger by welcoming those who can contribute to our society, grow our economy and assimilate into our great nation.
“He will always reject temporary, weak and dangerous stopgap measures that threaten the lives of hardworking Americans, and undercut immigrants who seek a better life in the United States through a legal pathway.”
Mr Trump’s remark reportedly came as lawmakers from both parties visited him to propose a bipartisan immigration deal.
Democratic Senator Richard Durbin had just been discussing US temporary residency permits granted to citizens of countries hit by natural disasters, war or epidemics, say US media.
According to the Post, Mr Trump told lawmakers the US should instead be taking in migrants from countries like Norway, whose prime minister visited him a day earlier.
Senator Lindsey Graham, a South Carolina Republican, was also in the meeting, but would not comment on the president’s reported slur.
The New York Times reported three weeks ago that Mr Trump had said Haitians “all have Aids” during a June meeting about immigration.
A backlash to his latest alleged remarks was swift.
Elijah Cummings, a Maryland Democratic lawmaker, tweeted: “I condemn this unforgivable statement and this demeaning of the office of the Presidency.”
Another black Democratic lawmaker, Cedric Richmond, said Mr Trump’s comments “are further proof that his Make America Great Again agenda is really a Make America White Again agenda”.
Mia Love, a Utah Republican and the only Haitian-American in Congress, demanded Mr Trump apologise for the “unkind, divisive, elitist” comments.
The National Association for the Advancement of Colored People (NAACP) accused the president of falling “deeper and deeper into the rabbit hole of racism and xenophobia”.
But the White House was apparently brushing off such criticism.
One Trump official was quoted by CNN as saying: “Though this might enrage Washington, staffers predict the comment will resonate with his base, much like his attacks on NFL players who kneel during the National Anthem did not alienate it.”
A spokesman for the Salvadorean embassy in Washington DC declined to comment.
In Thursday’s meeting, lawmakers reportedly proposed restoring so-called Temporary Protected Status (TPS) permits for certain countries, while offering $1.5bn (£1.1bn) for a wall that Mr Trump wants built on the US border with Mexico. (Courtesy BBC)