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.

On 4 June, the White House identified 19 countries - predominantly in Africa, the Middle East and the Caribbean - which would face full or partial immigration restrictions.

The guidance on Tuesday was issued by the 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 from the US from the travel ban nations.

The memo, which makes clear the sweeping restrictions being imposed, says: "This hold includes all form types and making any final decisions (approvals, denials) as well as completing any oath ceremonies."

Matthew Tragesser, a spokesman for UCSIS, confirmed the pause to the New York Times. He said: "The Trump administration is making every effort to ensure individuals becoming citizens are the best of the best. Citizenship is a privilege, not a right."

Immigration lawyers say that some clients have had citizenship hearings cancelled this week, including for migrants coming from Venezuela, Iran, and Afghanistan, according to ABC News.

The naturalisation ceremony is the culmination of up to five years of application processing. It typically features crowds waving miniature American flags as new citizens read their oath of allegiance to the United States.

The latest move from the White House comes as Trump increasingly blames migrants and refugees for causing what he terms the US's "social dysfunction".

The Trump administration has intensified its immigration crackdown after guardsmen Sarah Beckstrom, 20, was killed, and Andrew Wolfe, 24, seriously injured in the shooting last Wednesday in the national capital.

The suspect who has been arrested is originally from Afghanistan, and had travelled to the US under a programme that offered special protections to Afghans who had worked with US forces before the return to power of the Taliban.