William Hague urged Syrian opposition groups to build a united coalition against the "murderous" Assad regime, hours after failing to rule out an international military intervention.
The Foreign Secretary warned the country stood "on the brink" of all-out civil war in the wake of the massacre in Houla and the failure to implement international envoy Kofi Annan's peace plan.
Speaking from Istanbul, where he also met UN secretary general Ban Ki-moon, he said he had sought agreement on how the international community could help forces opposed to president Bashar Assad.
"My meetings with the Syrian opposition and the Turkish foreign minister were important and timely in the light of the sickening murders at el-Houla and other parts of Syria," he said. "We discussed the desperate need for a political solution in Syria and the importance of implementing the Annan Plan in full to ensure that a political transition happens quickly.
"We discussed what more the UK and international community could do to help Syria's opposition become a credible and effective alternative to president Assad's murderous regime. I emphasised the need to build a coherent and unified opposition that brings together different representative groups."
The meetings, which included Turkish foreign minister Ahmet Davutoglu, came as activists claimed there had been a further mass killing by pro-government militias. Some or all of the more than 100 brutal killings in Houla are believed to have been committed by pro-regime gunmen and are being investigated in an emergency session of the UN human rights body.
Earlier, amid reports of another mass killing by pro-government militias and increasing pressure for outside intervention, Mr Hague said a military response could not be ruled out.
"We are not ruling anything out," he told BBC Radio 4's Today. "But a military intervention in Syria, as I have always pointed out, would have to be on a vastly greater scale than was the case in Libya and it would have to enjoy broad international support. So we are not at that stage at this point now."
After meeting Mr Ban, Mr Hague said: "The secretary general and I share a deep concern that Syria is on the brink of a vicious civil war.
"We agreed that president Assad must fulfil his commitment to implementing the Annan Plan if this scenario is to be averted. We resolved to continue to work tirelessly to solve the crisis and will consider all options available in doing so."