US President Donald Trump has threatened to escalate his crackdown on immigration - pledging to "permanently pause migration" to the US from all "third world countries" as he hit out against his country's "refugee burden".

Trump's social media post came after he announced that a US National Guard member had died after a shooting in Washington DC - for which an Afghan national has been blamed.

He did not give further details or name which countries might be affected. Such a plan could face legal challenges and has already prompted pushback from UN agencies.

The president's announcements after Wednesday's fatal attack represent a further toughening of his stance towards migrants during his second presidency.

Among other moves, Trump has sought to enact mass deportations of migrants who entered the US illegally, to drastically cut the annual number of refugee admissions, and to end automatic citizenship rights that currently apply to nearly anyone born on US territory.

In the wake of Wednesday's shooting, Trump promised to remove from the US any foreigner "from any country who does not belong here". The same day, the US suspended processing all immigration requests from Afghans, saying the decision was made pending a review of "security and vetting protocols".

Then on Thursday, the US Citizenship and Immigration Services (USCIS) said it would re-examine green cards issued to individuals who had migrated to the US from 19 countries. The agency did not explicitly mention Wednesday's attack.

When asked by the BBC which countries were on the list, the USCIS pointed to a June proclamation by the White House that included Afghanistan, Cuba, Haiti, Iran, Somalia and Venezuela. There were no further details about what the re-examination would look like.

Trump's strongly worded two-part post on Thursday night went further still, pledging to "end all federal benefits and subsidies to noncitizens".

The US president wrote in a Truth Social post that this would "allow the US system to fully recover" from policies that had eroded the "gains and living conditions" of many Americans.

In the post, the president also blamed refugees for causing the "social dysfunction in America" and vowed to remove "anyone who is not a net asset" to the US.

The post, which Trump introduced as a "Happy Thanksgiving salutation", was filled with anti-immigrant language.

He said that "hundreds of thousands of refugees from Somalia were completely taking over the once great State of Minnesota" and took particular aim at the state's Democratic lawmakers.

"I will permanently pause migration from all Third World Countries to allow the U.S. system to fully recover," the president wrote.

The phrase "third world" is a term that was used in the past to describe poorer, developing nations.

The White House and USCIS have not yet given further details of Trump's plan, which Trump did not directly link in his post to Wednesday's attack.

The president had already imposed a travel ban on nationals of Afghanistan - and 11 other countries, primarily in Africa and Asia - earlier this year. Another travel ban targeting a number of majority-Muslim countries was enacted during his first term.

The UN responded to Trump's words by urging his administration to observe international agreements regarding asylum seekers.

"We expect all countries, including the United States, to honour their commitments under the 1953 Refugee Convention," the deputy spokesperson for the UN secretary general told Reuters.

The Trump response amounted to a "scapegoating" of migrants in the US, argued Jeremy McKinney, president of the American Immigration Lawyers Association.

Speaking to the BBC World Service's Newsday programme before Trump's latest comments, Mr McKinney highlighted that the attacker's motive was not known.

"These types of issues - they don't know skin colour, they don't know nationality," he said. "When a person becomes radicalised or is suffering some type of mental illness, that person can come from any background."

The flurry of announcements come after officials said that the suspect in the Washington DC shooting, Rahmanullah Lakanwal, had come to the US in 2021.

He travelled under a programme that offered special immigration protections to Afghans who had worked with US forces in the wake of the American withdrawal from Afghanistan.

Information is continuing to emerge about the extent of Mr Lakanwal's work alongside the Americans.

Mr Lakanwal previously worked alongside the CIA, the agency's current director has said.

He helped guard US forces at Kabul airport as thousands scrambled to escape Afghanistan before the Taliban took power, a former military commander who served alongside him told the BBC.

The father of five had been recruited to Unit 03 of the Kandahar Strike Force, nine years earlier.

His unit was known locally as Scorpion Forces, operating initially under the CIA but eventually for the Afghan intelligence department known as the National Directorate of Security.

Mr Lakanwal was a GPS tracker specialist, the former commander told the BBC, describing him as a "sporty and jolly character".

Mr Lakanwal would have been vetted by the US both at the time the he started his work alongside the CIA, and when he ultimately travelled to the US, according to a senior US official who spoke to CNN.

A childhood friend told the New York Times that Mr Lakanwal had experienced mental health issues after his work with his unit.