Rob Wainwright, who heads Europol, warned that “multiple thousands” of young Muslim men may have travelled to Syria to fight for al-Qaeda-linked groups.
He said many were using the internet to research their visits to the war zone and then to boast about their exploits.
In an interview with The Telegraph, Mr Wainwright said European Muslim men were being radicalised in Syria by terrorist groups including al-Qaeda-linked fighters.
He said the issue was a “top priority” for police forces across Europe.
One concern is that returning fighters, trained in the use of weapons and explosives, could carry out attacks in Britain.
“Syria is just the latest of a number of sort of areas of conflict zones that have acted in this way to radicalise people in the West, but it’s certainly the number one pull factor at the moment,” said Mr Wainwright.
“These are people that are coming from many countries in Europe. The UK is obviously part of the problem.”
Mr Wainwright said the location of Syria was helping young Muslims to travel to the area to fight Bashar Al-Assad’s regime.