William Shawcross, head of the Charities Commission, said groups raising money in Britain must be careful about where the funds go, The Daily Telegraph reported Friday.
"A lot of money is raised that goes to Syria, some of it undoubtedly goes to extremist groups," Shawcross said. "It is very hard for all organizations to determine that."
The Disasters Emergency Committee -- a coalition that includes well-known groups like Oxfam, the British Red Cross and Save the Children -- launched the Syria Crisis Appeal in March. The drive has raised 20 million pounds ($32 million).
The Charities Commission said imposters can use the names of legitimate charities to raise money. Peter Clarke, a former anti-terrorism chief with London's Metropolitan Police, suggested terrorists can set up front groups that look like charitable organizations, although he said there is no proof that has happened yet with Syrian aid.
Clarke warned that even well-intentioned groups may have trouble controlling what happens to money they have raised.
"Once you get into these very difficult, dangerous areas it is hugely difficult for charities to track the final destination of their funds," he said. "It is one of these 'fog of war' issues where stuff can be diverted."