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.
An estimated 93,000 people have died in Syria since the fighting began in 2011, with the US and UK governments saying they believe the government has used chemical weapons, which it denies.
The US announced on Friday that it would supply some rebels with direct military aid.
This followed the UK and France succeeding in getting the European Union to lift its ban on supplying arms to the country.
But Russia, a supporter of Mr Assad, has insisted it will continue supplying arms to the "legitimate government of Syria".
This has increased fears the conflict will escalate. Last week, 81 Conservative MPs wrote to Prime Minister David Cameron demanding a full House of Commons vote before any arms were sent.
But Mr Hague told BBC Radio 4's Today programme: "We have not taken any decision at this stage."
Of Syria, he added: "It's the worst human tragedy of our times. It's on a trajectory to get worse. I don't want to underestimate the severity and bleakness of this crisis."
Mr Hague said it was important to back "sensible" parts of the opposition, adding: "We really shouldn't be ruling out any options, and there are no palatable options at all. Of course it's not easy to take any decision to send arms into a conflict.
"It's also not easy to take a decision to allow people to be killed who are faced by much superior arms and who may be driven to radicalism and extremism by being placed in that situation."
Asked about the make-up of the rebel groups, the foreign secretary said: "There are certainly extremists on both sides."
But he continued: "We shouldn't fall into the trap of thinking that everybody on those sides is an extremist. There are a lot of decent Syrians."
The UK is currently sending "non-lethal" equipment, including humanitarian aid, to rebels.
President Assad's forces have recently seen some victories over the opposition, but Mr Hague said: "That doesn't mean the regime has re-established control over the whole country."
Writing in the Daily Telegraph, Boris Johnson said: "Surely to goodness it is time to recognise that no one can win this conflict, because it has become at least partly a religious conflict, between Sunni and Shia. No one can win that conflict because it is almost beyond reason."
He called for a "total ceasefire", adding: "We can't use Syria as an arena for geopolitical point-scoring or muscle-flexing, and we won't get a ceasefire by pressing weapons into the hands of maniacs."