US Secretary of State Hillary Clinton has slammed Russia and China as "despicable" for opposing UN action on Syria following a meeting of Western and Arab powers.
At an international conference of the 'Friends of Syria' group of nations, up to 70 delegates gathered to try to find a way to end the bloodshed in Syria.
The world leaders called on President Bashar al Assad to stop the killing of civilians and allow emergency aid into the country.
But the international efforts to resolve the crisis were criticised by Qatar and Saudi Arabia, who want Arab peacekeepers to enter Syria in order to enforce a ceasefire.
Western powers, including Britain and the US, favour a diplomatic approach to put more pressure on the Assad regime, including more economic sanctions.
As the concern over the humanitarian crisis grows, the Red Cross confirmed they, along with the Syrian Red Crescent, had been allowed into the besieged Syrian city of Homs to evacuate several wounded people.
Homs has been under constant bombardment by government forces for three weeks and hundreds of civilians are reported to have been killed.
On Friday aid workers entered the Babr Amr district to help those in need of medical assistance and managed to safely move seven people to a hospital in another part of the city.
It came as Mrs Clinton used strong language to denounce Russia and China for their attitudes toward the crisis in Syria.
The two nations were not in attendance at the Friends of Syria meeting in Tunisia, and earlier this month both voted against a UN resolution condemning the violence.
Speaking after the conference, Mrs Clinton said: "It's quite distressing to see two permanent members of the Security Council using their veto while people are being murdered - women, children, brave young men - houses are being destroyed.
"It is just despicable and I ask whose side are they on? They are clearly not on the side of the Syrian people."
While Moscow has backed calls for a ceasefire, it has criticised other Western countries for being "one-sided" in its approach.
However, there does appear to be evidence of disagreement over what exactly should be done to stop the killing of Syria's civilians.
Qatar and Saudi Arabia used the Tunis conference to called for an Arab force to impose peace.
Mrs Clinton used her speech to delegates to demand urgent humanitarian aid be allowed into Syria, adding that if the Assad regime did not co-operate, it would have "even more blood on its hands".
Directing her comments at Mr Assad, Mrs Clinton said: "You will pay a heavy cost for ignoring the will of the international community and violating the human rights of your people."
She also called on all nations to block Syrian assets, boycott oil from the country and consider closing embassies and consulates.
She added: "For nations that have already imposed sanctions, we must vigorously enforce them."
But later in the day the Saudi delegation quit the talks over what they called a lack of action, while the Saudi foreign minister even suggested that arming Syria's opposition fighters was an "excellent idea".
As Foreign Secretary William Hague arrived for the talks, he said it was important for Russia and China to no longer oppose the international community - and even suggested Beijing was reviewing its stance.
"I haven't seen that shift in Russia yet," he said. "I think the Chinese government is constantly assessing the position.
"So I hope... immediately they will change their position, but if not that, then they will steadily do so over time."
Mr Hague also said Britain would recognise the Syrian opposition as a "legitimate representative" of the people, and called the government "a criminal regime".
"We will intensify our links with the opposition. We, in common with other nations, will now treat them and recognise them as a legitimate representative of the Syrian people."
Also at the conference, France's foreign minister announced that the European Union would freeze the assets of Syria's central bank next week.
Alain Juppe told delegates the EU was planning on implementing the "strong new measures" in its meeting on Monday.
It is believed more than 7,000 people have now died in the uprisings against President Assad.
The draft conclusion of the meeting demanded that the Syrian authorities facilitate the delivery of emergency aid to areas under siege - including Homs, Deraa and Zabadani.
But the Syrian regime has refused to comply with such calls so far.
Hosting the summit, Tunisian president Moncef Marzouki told Sky News the world can no longer stand by and "watch the massacre" in Syria.
But the Tunisian leader ruled out a military solution, warning that foreign military intervention or arming the opposition could only lead to an escalation of violence.