UK Foreign Secretary William Hague has warned there is no "palatable option" for dealing with the crisis in Syria.
He told the BBC there were "extremists" supporting both President Assad's government and rebel forces, but said help would go towards "moderates".
But Mr Hague said the government had not "taken any decision" on whether to send arms to the opposition.
Meanwhile, London Mayor Boris Johnson has urged against "pressing weapons into the arms of maniacs".
The Syrian conflict is set to dominate the two-day summit of the G8 group of industrialised nations in Northern Ireland.