The US has halted the processing of all immigration applications linked to 19 countries already subject to a travel ban, according to an internal official memo seen by BBC partner CBS News.
Immigration agents have been told to "stop final adjudication on all cases", and pause naturalisation ceremonies for migrants on the cusp of gaining citizenship.
It comes amid reports that Donald Trump is considering expanding a June order restricting travel from 19 countries to 30.
It is the latest tightening of immigration rules after last week's shooting in Washington DC, which left one National Guard soldier dead and another in a critical condition.
The suspect is an Afghan national living in the US.
