Britain and other EU countries could well be faced with having to deal with many more ISIS jihadists returning from IS territories. Trump has stated today that he doesn't want them on USA soil and their home countries should take them back and deal with them.
Alex Younger also told of his concern about jihadists returning to Europe with "dangerous"
skills and connections.
The president's tweets come amid a controversy in the UK over the case of Shamima Begum, one of three schoolgirls who left London in 2015 to join IS.
Trump administration officials have also told the Sunday Telegraph newspaper that they fear some of the detained soldiers would put European countries at risk unless they are brought to justice.
Mr Trump's remarks echo those made by the UK's foreign intelligence chief on Friday, who warned that the Islamic State group is reorganising for more attacks despite its military defeat in Syria.