The risks that UK-supplied weapons could fall into the hands of jihadists in Syria may well be outweighed by the gamble of not doing more to end the crisis, William Hague said.
No decision had been taken to arm the opposition to Bashar Assad's regime, the Foreign Secretary said amid British and French pressure for a European Union arms embargo to be lifted.
Any shipments of weapons would have to be "very carefully controlled" but there were "even greater risks" posed by continued violence, he said.
"If we did that it would have to be very carefully controlled in terms of what we would actually send, how we would monitor what was sent and the guarantees that would be needed from the people they were sent to," he told the Murnaghan programme on Sky News.
"There are risks, of course, which is why we haven't so far sent lethal equipment to the opposition.
"But if this crisis goes on worsening in the way that it is in the coming weeks and months, there are even greater risks that have to be weighed against that: the risks of international terrorism and extremism taking root in Syria; the risks of Lebanon, Iraq, Jordan being destabilised; and the risks of extreme humanitarian distress.
"As ever in foreign policy, you have to weigh some risks against other risks."
Prime Minister David Cameron has warned the UK "might have to do things our own way" if it fails to secure a relaxation of the embargo at a meeting in May.
It was relaxed last month to allow the supply of non-lethal military equipment and the UK announced it was sending body armour and armoured cars among other equipment.
Mr Hague said there was a "strong case" for lifting the embargo on the opposition or at least fundamentally amending it, but several member states remain opposed.
He said the UK was "reluctant" to act alone "but we will do that if it is necessary.
Mr Hague added: "I stress we have taken no decision at the moment to send arms to anybody in Syria or what we will do at the end of May when the current sanctions regime comes to an end.
"But we are clear, with France, that we need to keep doing more, that this situation us deteriorating rapidly.
"We now have nearly 1.1 million refugees, hundreds of thousands of people dead, other countries being destabilised.
"Without a diplomatic and political breakthrough - though we are working on that all the time - we will have to do more to try to change the calculations of the Assad regime."
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