Britain has not ruled out any option to save lives in Syria, William Hague said in a warning to the regime not to deploy chemical or biological weapons.
The Foreign Secretary said president Bashar Assad should not "doubt our resolve" to react strongly if the military escalate the conflict.
At a Friends of Syria meeting in Morocco, he urged more countries to rally behind the opposition and warned that time was short to secure transition.
France, which with the UK has already formally recognised Syrian National Coalition as the government-in-waiting, said more than 100 nations present had now followed suit.
The apparent breakthrough came after Mr Hague told the gathering he was "deeply disappointed by the failure of the international community" to put sufficient weight behind it.
"Syria needs a political transition leading to an inclusive new government, with full executive authority," he said in his opening remarks.
"This is the course most likely to achieve stability in the country.
"But in the United Kingdom we do not rule out any option to save lives. The Assad regime should not doubt our resolve, or miscalculate how we would react to any use of chemical or biological weapons against the Syrian people.
"The next few months will determine whether a peaceful political transition can be agreed, or if Syria is to face more bloodshed."
Mr Hague said the UK would add another £1 million in communication support to the £7 million of non-lethal help it has already supplied. And he called on other countries to boost aid spending to ease the humanitarian crisis faced by half a million refugees and up to three million people displaced by the violence.