The Foreign Secretary William Hague has told the Commons that some UK nationals could be fighting for the extremists in Iraq.
It's thought there are around 400 jihadists from the UK currently fighting in Syria but the government has calculated that some of those will now be among the ISIS extremists who have taken over many cities in northern Iraq.
The group - linked to Al Qaeda - are strong in eastern Syria and last week overran national forces in the north of Iraq.
We already knew that there were an estimated 400 British nationals and "other UK-linked individuals" fighting in Syria - but it is significant that the government has linked them to the latest fighting - particularly at a time when there are claims the extremists have carried out executions in the areas they control.
The Foreign Secretary said the fighters from the UK could present a risk if they return here and he warned that they risked having their British passports removed or having their leave to remain the UK cancelled.