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More than 300 people admitted to the US as refugees are being investigated by the FBI for potential terror-related activities, says the top law official.

Attorney General Jeff Sessions pointed to this FBI probe to justify the new executive order banning travel from six mainly Muslim countries.

The order, which puts a 90-day travel ban on Iran, Libya, Syria, Somalia, Sudan and Yemen, begins on 16 March.

But Mr Sessions did not say how many of the 300 came from the banned countries.

Nor did he say what the alleged offences are or how many may face charges.

A spokeswoman for the American Civil Liberties Union told the ³ÉÈË¿ìÊÖ it was an assertion that "leaves many of us scratching our heads".

"The Trump administration has offered up no proof behind this assertion, making it impossible to evaluate this claim without more information."

The president signed the new order on Monday, building upon a previous executive order on immigration that was blocked in federal court.

The fact sheet attached to the order says: "The Federal Bureau of Investigation has reported that approximately 300 persons who entered the United States as refugees are currently the subjects of counterterrorism investigations."

The new directive includes a 120-day ban on all refugees but it also lifts a previous temporary ban on all Syrian refugees.

White House Press Secretary Sean Spicer spoke briefly on Monday about federal investigations into refugees in the US as part of a counterterrorism programme.

The number of individuals - 300 - is high. But it's not uncommon for the FBI to conduct investigations into people suspected of terrorism. It's part of a broader counterterrorism programme that's designed to prevent more attacks in this country.

At any given time, the FBI has hundreds of open investigations, according to Rand Corporation's Kim Cragin, who worked on a 2015 report about the FBI.

Most of these investigations turn up nothing, and the cases are closed. At this point it's not clear how much of a danger these particular individuals pose - or what the investigations will show.

"The majority of the people convicted in our courts for terrorism-related offences since 9/11 came here from abroad," Mr Sessions said at a news conference on Monday.

"In fact today - more than 300 people, according to the FBI - who came here as refugees, are under an FBI investigation today for potential terrorism related activities."

The homeland security boss John Kelly was asked by CNN how many of the 300 came from the six countries but said he did not know, and he said he could not comment on the nature of the alleged and potential offences.

The FBI has yet to comment on the reported ongoing investigation and it is unclear what is considered to be terror-related activities.

In 2011, Barack Obama tightened security measures for Iraqis after two men were arrested on terror charges.

Iraqi natives Waad Ramadan Alwan and Mohanad Shareef Hammadi were arrested on charges of attempting to send weapons and money to Iraq to support al-Qaeda there.

They also admitted to using homemade bombs against US troops while living in Iraq.

The pair lived in Bowling Green, Kentucky, but were never accused of planning or attempting to carry out an attack.

White House senior aide Kellyanne Conway attempted to reference the two men's arrests in defending Mr Trump's initial travel ban, but came under fire after she blamed them for a massacre that never happened.