London, AP—British intelligence officials will be unable to keep track of hundreds of young Britons returning after fighting with militants in Syria and Iraq, a former spymaster has warned.

Fears about returning radicals have been heightened by a video released online that showed three men identified as British exhorting compatriots to join militant group the Islamic State of Iraq and Syria (ISIS).

Two of the militants have been identified by their families as 20-year-olds from Cardiff, Wales.

Officials estimate between 400 and 500 Britons have traveled to Syria to fight, and some may have crossed into Iraq.

Richard Barrett, a former head of counterterrorism at the MI6 intelligence agency, told the BBC that as many as 300 might already have returned to Britain and that keeping track of them all was an impossible task.

“Clearly they’ll have to prioritize and they’ll have to choose those that they think are likely to pose the greatest risk,” he said.

Cressida Dick, Assistant Commissioner of London’s Metropolitan Police, said authorities were “alert to the fact that people may come back and they may have military training.”

“They may seek—although it’s very difficult—to smuggle weaponry here and they may seek to cause violence or to encourage others to cause violence,” she told the BBC.

Prime Minister David Cameron’s office said on Monday that the government was pressing YouTube and other online services to remove the video and others like it, as officials warned that publicity over the recruitment videos could encourage more young British Muslims to join the fight.

Ahmed Muthana, whose son Nasser appeared in the video, said the young man had been brainwashed.

“He was born here in the hospital down the road. He has been educated here,” Muthana told the Guardian newspaper. “He has betrayed Great Britain,” he added.