Russian foreign minister Sergei Lavrov has accused the United States of justifying the use of terrorism against the government of Syrian president Bashar al-Assad.
Lavrov slammed the United States' reaction to rebel assaults and to the suicide bomb attack on the state security headquarters in Damascus which targeted top members of Assad's inner circle.
Lavrov was responding after US State Department spokeswoman Victoria Nuland said the attacks were not surprising given the Syrian government's violent conduct.
Lavrov said: "This is a direct endorsement of terrorism. How are we supposed to understand that?
"This is a sinister position. I cannot find words to express our attitude towards that," Russai Today reported.
He also criticised the United Nations Security Council for failing to condemn the attacks.
Syrian rebels or al-Qaida fighters?
As the fight between anti-Assad rebels and government forces accelerates, Lavrov questioned progress made by the Syrian opposition on the ground and hinted that al-Qaida fighters were behind most of the territorial gains.
The minister said that recent reports suggested that the Syrian opposition was in control of border posts on the Iraqi and Turkish frontiers with Syria. There were also reports of Turkish property being looted, he added.
He quoted some sources saying that the posts had fallen to al-Qaida fighters - not Free Syrian Army soldiers. Russian diplomats were trying to verify the information.
"If taking territories by terrorists is supported by our partners, we would like to ask them what their position on Syria is. What do they want to achieve in this country?" Lavrov said.
Syrian activists said that the army was deploying thousands of soldiers from the border with Turkey to the key Syrian city of Aleppo, which has been in the grip of fierce fighting for over a week.
The capital of Damascus has also witnessed continued clashes with reports claiming that government forces have repelled the rebels.
In a further development, activists have posted footage online purportedly showing the victims of a massacre in the Qaboun district of the city.
The Local Coordination Committees in Syria , an activist group, issued a statement on its Facebook page which said: "The regime's army committed a new massacre against unarmed civilians who had hidden in a shelter due to fear of the shelling.
"After shelling, which had targeted the neighbourhood for the past five days, regime forces detained more than 150 people from one of the shelters. The detainees were taken to military police and threatened with death. Their bodies were found in the area of Tishreen - some were executed and others were killed [by] the shelling."