The UN's human rights chief has warned that Syria could descend into all-out civil war unless UN member states demand an independent inquiry into the Houla massacre.
In a statement read out to the UN's human rights council (HRC), Navi Pillay reasserted her belief that President Assad's regime should be referred to the International Criminal Court (ICC).
She also appealed for the other countries attending the emergency session to rally behind Kofi Annan’s six-point pace plan to avoid “fully-fledged conflict”.
Earlier, a draft resolution to be put before the HRC condemned "in the strongest possible terms such an outrageous use of force against the civilian population which constitutes a violation of applicable international law".
However, Syria's ambassador to the UN, Fayssal al-Hamwi, said the resolution was designed to ignite civil war in the country, dismissing the draft as "full of hate".
Hamwi also condemned the Houla massacre, but placed the blamed for the killing on "armed terrorists”.
The massacre, in which 108 people, including 49 children, were killed, sparked international outrage, with some states now discussing the prospect of military intervention in the restive state.
"I am appalled by the atrocities committed in Houla," said Pillay.
"There is a need for prompt, independent and impartial international investigations into all serious human rights violations in Syria, including those that have occurred in Houla."
William Hague welcomed the resolution saying that the acts committed by the Syrian regime in Houla "may amount to crimes against humanity and other international crimes."
They "demonstrate a pattern of widespread and systematic attacks against civilian populations" he said.
"Evidence from the UN supervision mission in Syria and independent witnesses has confirmed that security forces shelled the village of el-Houla and that government militia then went house to house slaughtering entire families without compassion or mercy.
"These atrocities must be investigated, and those responsible must be held to account. A special and independent investigation into el-Houla is essential - including to ensure that evidence is gathered and preserved for future justice processes.
"Britain will support these efforts, and will continue to prepare the ground for a referral of the situation in Syria to the International Criminal Court."
On Friday morning, Hague said intervention in Syria would have to take place on a much larger scale than the operations in Libya.
The foreign secretary said the government had not ruled out any options, which might also include supplying rebels in Syria with arms.
Later on Friday, Hague urged Syrian opposition groups to build a united coalition against the "murderous" Assad regime.
The minister 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 also 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 Ban today, 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."
Reuters has reported that a Russian cargo ship docked at the Syrian port of Tartus last weekend. Western officials Believe was ship carrying weapons for the regime.
The following video purports to show a protest taking place in Al Qousair, Homs during the funerals of several people recently killed in the violence: