White House officials have refused to clarify Donald Trump and Jeff Sessions’ unsubstantiated claim that more than 300 refugees are being investigated for terrorism offences.

The administration repeatedly pedalled the statistic on Monday to justify a revised executive order restricting travel to the US for people from six Muslim-majority countries.

But officials will not say how many of the 300 come from the banned countries and if any of the 300 have been charged.

“The Attorney General has reported to me that more than 300 persons who entered the US as refugees are currently the subjects of counter terrorism investigations by the FBI,” said Mr Trump on Monday.

Mr Sessions, the Attorney General, also said: “More than 300 people, according to the FBI, who came here as refugees, are under an FBI investigation today for potential terrorism-related activities.”

An unidentified senior administration official also said in a teleconference news briefing that “approximately” 300 individuals were being probed.

“Not a small number,” they added.

“That’s a tremendous administrative burden of manpower and resources.”

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But The New York Times and The Washington Post reported that Homeland Security and Justice Department officials are declining to give any further details.

The Times' White House correspondent Glenn Thrush said: “Officials flatly refused to say how many come from banned countries.”

A Post editorial added: “Here’s the problem. Without context, this 300 figure is meaningless.”

The article goes on to claim that, according to the Pew Research Center, 300 represents less than 1 per cent of the average number of refugees entering the US every year since 1980.

The Post reported last May that around 10 terrorism-related cases since 2009 have involved refugees.