The move - part of the Trump administration's efforts to curtail illegal immigration - closes a loophole that had allowed visa holders and visitors to apply for a green card while still in the US.

Critics of the policy say the longstanding system allowed families to stay together during the lengthy application process. The new method could also make it difficult or impossible for some immigrants who leave the country in hopes of gaining a green card - which grants permanent residence in the US - to return. The USCIS policy memo states that people such as students, temporary workers or people on tourist visas need to go through the Department of State from outside of the US.

"When aliens apply from their home country, it reduces the need to find and remove those who decide to slip into the shadows and remain in the US illegally after being denied residency", USCIS said, making the system "fairer and more efficient". The Department of Homeland Security, which oversees USCIS, wrote on X: "The era of abusing our nation's immigration system is over". "We're returning to the original intent of the law to ensure aliens navigate our nation's immigration system properly", USCIS spokesman Zach Kahler said. "From now on, an alien who is in the US temporarily and wants a green card must return to their home country to apply, except in extraordinary circumstances."

Kahler said the policy would allow the immigration system "to function as the law intended instead of incentivising loopholes" and that visits "should not function as the first step in the green card process". It is unclear whether pending green card applications will be affected. A spokesperson for the USCIS told the BBC that as the policy was rolled out, "people who present applications that provide an economic benefit or otherwise are in the national interest will likely be able to continue on their current path". "Others may be asked to apply abroad depending on individualised circumstances," they said. Obtaining a green card is a multi-step process that can take months to several years.

There are currently more than a million legal immigrants waiting for approval on their adjustment-of-status green card applications, according to the Cato Institute's director of immigration studies. Kahler argued that following the law allowed for the majority of cases to be handled by the US State Department at consular offices abroad and freed up USCIS resources to focus on processing other cases that fall under its purview - such as visas for victims of violent crime and human trafficking, and naturalisation applications, among others. (BBC)