According to an internal official memo, the US has halted the processing of all immigration applications from 19 countries that are already under travel ban.
Immigration agents have been told to "withhold final adjudication on all cases" and to suspend naturalization ceremonies for immigrants on the verge of citizenship.
This comes amid reports that Donald Trump is considering extending a June order that restricted travel from 19 countries to 30.
This is the latest tightening of immigration rules following last week's shooting in Washington, D.C., in which one National Guard soldier was killed and another is in critical condition.
The suspect is an Afghan citizen living in the US.
On June 4, the White House identified 19 countries—primarily in Africa, the Middle East, and the Caribbean—that would face full or partial immigration restrictions.
The guidance was issued on Tuesday by US Citizenship and Immigration Services (USCIS), which last week also halted all asylum decisions and said it would re-examine green card visas issued to migrants arriving in the US from countries under the travel ban.
The memo, which explains the major restrictions being imposed, states: "This halt includes the completion of all forms and any final decisions (approval, denial), as well as any oath ceremony."
UCSIS spokesperson Matthew Trageser confirmed the halt to the New York Times. He said: "The Trump administration is making every effort to ensure that those who become citizens are the best possible. Citizenship is a privilege, not a right."
Immigration lawyers say citizenship hearings have been canceled for some clients this week, including migrants from Venezuela, Iran, and Afghanistan.
The naturalization ceremony is the culmination of up to five years of application processing. It typically features crowds waving small American flags as new citizens recite an oath of allegiance to the United States.
This new move by the White House comes at a time when Trump is blaming migrants and refugees for "social dysfunction" in America.
The Trump administration has intensified its immigration crackdown following the shooting at the National Capitol last Wednesday that killed Guardsman Sarah Beckstrom, 20, and seriously injured Andrew Wolfe, 24.
The suspect arrested is originally from Afghanistan and came to the US under a program that provided special protections to Afghans who served with the US military before the Taliban returned to power.