The Trump administration on Friday announced that any foreigners in the US seeking to adjust their visa status must now depart the country first, and then apply through a US embassy abroad.

The move is a profound policy shift that could affect tens of millions of visa holders in the US, including international students, temporary workers, refugees on special visas, and even those with American spouses.

"We’re returning to the original intent of the law to ensure aliens navigate our nation’s immigration system properly. 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," US Citizenship and Immigration Services (USCIS) spokesperson Zach Kahler said in a statement.

It is understood that the policy goes into effect immediately, but it remains unclear what "extraordinary" would entail. Immigration officers, as with customs and border officers, are traditionally given broad discretion to act.

"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," he added.