Trump did not identify any countries by name or explain what he meant by third-world countries or "permanently pause".
He said the plan would include cases approved under former president Joe Biden's administration.
"I will permanently pause migration from all Third World Countries to allow the US 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," he said on his social media platform, Truth Social.
White House and US Citizenship and Immigration Services did not immediately respond to Reuters' requests for comment.
Trump's comments followed the death of a National Guard member who had been shot near the White House in an ambush investigators say was carried out by an Afghan national.
Earlier, officials from the Department of Homeland Security officials said that Trump had ordered a widespread review of asylum cases approved under Biden's administration and Green Cards issued to citizens of 19 countries.
The alleged gunman was granted asylum this year under Trump, according to a US government file seen by Reuters.
The Afghan immigrant, identified as 29-year-old Rahmanullah Lakanwal, had entered the US in 2021 under a resettlement program.
The resettlement program was set up by Biden after the US military withdrawal from Afghanistan in August 2021 that led to the rapid collapse of the Afghan government and the country's takeover by the Taliban.
In a separate post prior to the announcement, Trump claimed that hundreds of thousands of people poured into the US totally "unvetted and unchecked" during what he described as the "horrendous" airlift from Afghanistan.
The US Citizenship and Immigration Services on Wednesday stopped processing all immigration requests relating to Afghan nationals indefinitely.
"These goals will be pursued with the aim of achieving a major reduction in illegal and disruptive populations," Trump said.
"Only REVERSE MIGRATION can fully cure this situation."
The Trump administration also said it would review green cards issued to all foreigners from countries "of concern".
Joseph Edlow, director of the US Citizenship and Immigration Services (USCIS), said on X that he had directed "a full scale, rigorous re-examination of every Green Card for every alien from every country of concern" at the direction of Trump.
He did not initially specify which nationalities would be targeted.
A green card allows foreign nationals to live and work permanently in the United States.
A USCIS news release cited "19 high-risk countries" without listing them, but linked to a June presidential decree announcing entry bans for people coming from 12 countries, including Afghanistan, Iran and Somalia, and a partial restriction of entry for nationals of a further seven countries, including Cuba and Venezuela.
with DPA
