President Donald Trump has said he will "permanently pause" all immigration from what he called "third world countries" and demanded a program of "reverse migration" as he intensified his rhetoric after the National Guard shooting in Washington, D.C.

Trump offered few details as he disparaged and vowed to remove millions of migrants in the U.S., in a lengthy social media post late on Thanksgiving that came hours after he confirmed the death of National Guard troop Sarah Beckstrom, 20, in the shooting.

Officials have said that Wednesday`s attack on two troops was carried out by an Afghan national who worked with a CIA-backed group during the long war in Afghanistan. The incident has served as a catalyst for Trump to escalate his anti-immigrant rhetoric into pledges that would likely face court challenges if enacted and further undermine America`s global standing as a nation welcoming to immigrants.

"I will permanently pause migration from all Third World Countries to allow the U.S. system to fully recover, terminate all of the millions of Biden illegal admissions, including those signed by Sleepy Joe Biden`s Autopen, and remove anyone who is not a net asset to the United States," Trump wrote on Truth Social.

It was not clear exactly which countries he was referring to, with the phrasing used in the past to refer to poorer nations. The Department of Homeland Security referred a reporter from the Reuters news agency to the 19 countries listed in a travel ban in June, mainly in Africa and the Middle East.

The comments drew swift criticism from two U.N. agencies, which on Friday urged the U.S. to continue to accept asylum seekers.

The president also threatened to "end all federal benefits and subsidies to noncitizens of our country" and to "denaturalize migrants who undermine domestic security."

In remarks that will cause alarm among migrant advocacy and civil liberties groups, Trump said the government would deport any foreign national who was "non-compatible with Western Civilization."

He added: "Only REVERSE MIGRATION can fully cure this situation."

The Trump administration also said Thursday that the government would reexamine the status of Green Card holders from 19 countries "of concern," including Afghanistan.

In a subsequent post, Trump said that "hundreds of thousands" of Somali migrants were "completely taking over the once great state of Minnesota."

Trump has previously threatened action against Somalis and last week said he would end temporary protective status - which prevents deportations to dangerous countries - for Somali migrants in Minnesota, many of whom have fled a brutal civil war in the east African country.

It`s unclear how many people this would affect but a report made for Congress in August put the number of people covered by the program nationally at 705. The president also attacked Minnesota Gov. Tim Walk as "retarded" and said Rep. Ilhan Omar, D-Minn, who was born in Somalia, "probably came into the U.S.A. illegally" and is from a "decadent, backward, and crime ridden nation."

Trump`s threats, if enacted by legislation or executive orders, are likely to be challenged in the courts. In his first term, Trump banned people from several majority-Muslim countries in the Middle East and Africa from entering the U.S. This was challenged but eventually the Supreme Court ruled 5-4 in favor of the ban`s legality.

Migrant advocacy groups have called for calm and warned against using the D.C. attack to call for a wider crackdown on immigration or to remove the rights of Afghan residents.

"Using this horrific attack as an excuse to smear and punish every Afghan, every refugee, or every immigrant rips at something very basic in our Constitution and many faiths: the idea that guilt is personal, not inherited or collective," the Council on American-Islamic Relations said in a statement early Friday.

U.N. human rights office spokesperson Jeremy Laurence told a Geneva press briefing: "They are entitled to protection under international law, and that should be given due process."

U.N. refugee agency spokesperson Eujin Byun said: "When people who need protection arrive in their territory, they have to have a due process of asylum. And then they have to have access to territory," she said, adding that the overwhelming majority of refugees are law-abiding members of the host community.

The term "third world" originated in the Cold War era to describe a country that wasn`t aligned with the western NATO alliance or the USSR and the Warsaw Pact. This later evolved into shorthand for describing economically underdeveloped nations, particularly ones with high levels of poverty.

The term has been used to describe several African nations, but until the late 20th century was also attached to descriptions of China.

Economists and health experts have for years said the phrase is inaccurate, derogatory and outdated. The World Bank and other global institutions no longer use the phrase and some have suggested also avoiding its successor, "developing countries."