Russian Foreign Minister Sergei Lavrov has accused Washington of providing weapons to the Syrian opposition for their fight against pro-government forces.
Lavrov made the allegation on June 13 in Tehran where his talks with Iranian officials included the deteriorating conflict in Syria. (Editor's note: Lavrov's comments were later reported to have been mistranslated.
On June 12, U.S. Secretary of State Hillary Clinton accused Russia of dramatically escalating violence in Syria by providing President Bashar al-Assad's regime with shipments of heavy weapons, including sophisticated attack helicopters.
"We have confronted the Russians about stopping their continued arms shipments to Syria," she said. "They have from time to time said that we shouldn't worry, everything they're shipping is unrelated to their actions internally. That's patently untrue."
U.S. State Department spokeswoman Victoria Nuland declined to reveal the source of Clinton's information, but she said Clinton was referring to new helicopters that are still en route to Syria from Russia -- not Syria's existing helicopter fleet.
Russian President Vladimir Putin has said previously that Russia did not send weapons to Syria "that could be used in a civil conflict."
Meanwhile, a senior United Nations official has warned that Syria in now in a state of a civil war.
Herve Ladsous, the United Nations' peacekeeping chief, said a "massive increase in the level of violence" indicated the nature of Syria's conflict has changed.
"Clearly what is happening is that the government of Syria has lost some large chunks of territory, several cities, to the opposition and wants to retake control," Ladsous said.
Ladsous is the highest-ranking UN official to publicly declare Syria is in the midst of civil war. UN Secretary-General Ban Ki-moon said last week that he thought civil war was "imminent."
French Foreign Minister Laurent Fabius on June 13 also described the conflict in Syria as "civil war."
Fabius said France planned to ask the United Nations Security Council to make a UN-backed peace place for Syria obligatory by invoking the UN's Chapter 7 provision.
A Chapter 7 resolution in the Security Council can authorize the use of force, and is unlikely to be backed by Russia and China.
Syria's Foreign Ministry denied it was in a state of civil war, saying the country was facing "an armed conflict to uproot terrorism."
UN observers inspect a residential area with Free Syrian Army gunmen in the Talbisah area of Homs.
Meanwhile, Sausan Ghosheh, spokeswoman for the UN mission in Syria, says monitors were attacked on June 12 while trying to reach the besieged opposition-held town of Al-Haffa.
"The United Nations observers tried to reach Al-Haffa but we were confronted with angry crowds that surrounded our cars and stopped us from proceeding forward," Ghosheh said.
"They then started hurling stones and metal rods at our vehicles. The UN observers turned back. As we were leaving the area, three vehicles that were heading towards Idlib were fired upon. The source of fire is still unclear."
Opposition activists blamed regime loyalists for the attack on the monitors.
Ladsous, the UN peacekeeping chief, said he believed the attack was "deliberate."
Government forces have been reinforcing their encirclement of Al-Haffa and shelling the town for a week.
The UN monitors have been trying to investigate the worsening violence there since June 7.
The UN monitors previously accused government troops and pro-regime militia of massacring civilians in the village of Houla -- including scores of women and children.
On June 12, the UN added Assad's regime to its "list of shame" for killing and torturing children.
"In Syria, children as young as 9 years of age were victims of killing and maiming, detention, torture, arbitrary arrests and were used as human shields by the Syrian government forces, including the Syrian armed forces, the intelligence forces, and the Shabiha militia," said Radhika Coomaraswamy, the UN secretary-general's special representative on children in armed conflicts.
"And, in addition, these forces have regularly raided and used schools as military bases and detention centers."
The London-based Syrian Observatory for Human Rights says at least 72 people were killed across Syria on June 12 -- most of them civilians. It says more than 14,100 have been killed in the uprising since Damascus launched a deadly crackdown against protesters 15 months ago. The tolls could not be independently confirmed.
With reporting by Reuters, AP, AFP, and the BBC