Donald Trump said he would suspend migration from what the US leader called "third world countries", a day after an Afghan national allegedly shot two National Guard soldiers in Washington, killing one.

His angry post, which also threatened to reverse "millions" of admissions granted under his predecessor Joe Biden, marked a new escalation in the anti-migration stance of a second term that has been dominated by Trump's mass deportation campaign.

Trump had earlier announced that Sarah Beckstrom, a 20-year-old West Virginia National Guard member deployed in Washington as part of his crackdown on crime, had died from her wounds.

The FBI has launched an international terror investigation as new details emerged about the alleged gunman, a 29-year-old Afghan national who had served with US troops in Afghanistan.

The shooting on Wednesday, which officials described as an "ambush-style" attack, brings together three politically explosive issues: Trump's controversial use of the military at home, immigration, and the legacy of the US war in Afghanistan.

"I will permanently pause migration from all Third World Countries to allow the US system to fully recover," Trump wrote on social media on Thursday.

He also threatened to reverse "millions" of admissions granted under his predecessor Joe Biden, and to "remove anyone who is not a net asset to the United States."

In a Thanksgiving video call with US troops Trump said: "I want to express the anguish and the horror of our entire nation at the terrorist attack yesterday in our nation's capital."

He linked the shooting and his decision to send hundreds of National Guard troops to the city.

"If they weren't effective, you probably wouldn't have had this done," he said, adding: "Maybe this man was upset because he couldn't practice crime."

Joseph Edlow, Trump's director of US Citizenship and Immigration Services, said on Thursday that he had ordered a "full scale, rigorous reexamination of every Green Card for every alien from every country of concern."

His agency later pointed to a list of 19 countries – including Afghanistan, Cuba, Haiti, Iran and Myanmar – facing US travel restrictions under a previous order from Trump in June.

The Trump administration had earlier ordered an immediate halt to the processing of immigration applications from Afghanistan.

In the wake of Wednesday's shooting, US Defence Secretary Pete Hegseth announced 500 more troops would deploy to Washington, bringing the total to 2,500.

Trump has deployed troops to several cities, all run by Democrats, including Washington, Los Angeles and Memphis.

The move has prompted multiple lawsuits and allegations of authoritarian overreach by the White House. (AFP)