Hillary Clinton has claimed Russia's policy on Syria could lead to civil war in the country, as the United States' ambassador to the UN said the Assad regime's report on the Houla massacre was "a blatant lie".
The US Secretary of State bluntly criticised Russian backing of President Bashar al-Assad, during an address to students in Denmark.
Mrs Clinton said the Syrians were unlikely to listen to the US but added: "They will listen, maybe, to the Russians, so we have to keep pushing them".
She also criticised Russia's position on Syria, saying: "I have been telling them their policy is going to help to contribute to a civil war".
– Hillary Clinton, US Secretary of State
The Russians keep telling us they want to do everything they can to avoid a civil war because they believe that the violence would be catastrophic.
They often ... liken it to the equivalent of a very large Lebanese civil war and they are just vociferous in their claim that they are providing a stabilising influence. I reject that.
I think they are in effect propping up the regime at a time when we should be working on a political transition.
Mrs Clinton's claims came after it emerged that a Russian cargo ship carrying weapons had docked in Syria, a day after 108 people, including 34 children, were massacred in Houla.
The US Ambassador to the United Nations Susan Rice said the Russian shipment of arms to Syria was "reprehensible" but admitted it did not violate any laws.
– US Ambassador Susan Rice
With respect to the reported docking of a ship carrying Russian arms, this is obviously of the utmost concern.
It is not technically obviously a violation of international law ... but it's reprehensible that arms would continue to flow to a regime that is using such horrific and disproportionate force against its own people
Ms Rice said Russia's veto-wielding membership of the Security Council would not prevent possible future intervention in Syria. Some countries would consider bypassing Chinese and Russian opposition if violence worsened in the country, she said.
Members of this Council and members of the international community are (then) left with the option only of having to consider whether they're prepared to take actions outside of the Annan plan and the authority of this Council.
Our Middle East Correspondent John Ray, said that mass graves were still being dug in Houla, with villagers saying that the killers came from across the field.
The US ambassador to the UN also dismissed the Syrian Government's investigation into the massacre in Houla calling it "another blatant lie", adding that "there is no factual evidence ... that would substantiate that rendition of events".
The Prime Minister convened a meeting of the National Security Council today to discuss the growing possibility of all-out civil war in Syria.
– Downing Street spokesman
The Prime Minister wants to be sure that we are doing all we can to bring an end to the violence in Syria and to support the process of transition.
The National Security Council considered all the options on the table. Our goal is a peaceful political transition and our focus remains on the Annan plan but the Assad regime should not make the mistake of believing that we will abandon the Syria people.