Hillary Clinton has warned that Russia is sending attack helicopters into Syria, as the United Nations peacekeeping chief says the country is now in a full-scale civil war.
The US Secretary of State expressed concerns over reports that Russia is supporting Syrian President Bashar Assad's regime, which could fuel tensions further.
She said: "We have confronted the Russians about stopping their continued arms shipments to Syria.
"They have, from time to time, said that we shouldn't worry - everything they are shipping is unrelated to their (the Syrian government's) actions internally. That's patently untrue.
"And we are concerned about the latest information we have that there are attack helicopters on the way from Russia to Syria, which will escalate the conflict quite dramatically."
Her comments come as the UN's Herve Ladsous said violence in the country is escalating.
Asked if the situation in Syria warrants civil war, he said: "Yes I think we can say that.
"Clearly what is happening is that the government of Syria lost some large chunks of territory, several cities to the opposition, and wants to retake control.
"There is a massive increase in the level of violence."
He also referred to the use of helicopters in the government's attempts to recapture districts taken by the opposition.
Mr Ladsous said: "Now we have confirmed reports of not only the use of tanks and artillery but also attack helicopters.
"This is really becoming large-scale."
He is the first senior UN official to declare the conflict a civil war.
Moscow has consistently rejected the use of outside force to end the uprising against Assad's 40-year regime, and insists that any arms it supplies are not being used to quell anti-government dissent.
The two countries have a long-standing military relationship, with Syria hosting Russia's only naval base on the Mediterranean Sea.
If Russia is sending Syria attack helicopters, it would deal a serious blow to efforts to starve government troops of supplies.
The town of Haffa has been one of the areas worst-hit by the escalation in violence.
Syrian Free Army rebels withdrew from the area following bombardment by government troops, a rebel spokesman said.
Angry crowds blocked UN observers from reaching area.
Monitors said they were met with gunfire and their vehicles had stones and metal rods thrown at them, although no one was injured.
The Red Cross has said the situation is deteriorating in several parts of Syria simultaneously making it impossible to respond to all humanitarian needs at once.
Troops also stepped up attacks on the central city of Homs and its suburbs , pounding rebel positions.
But Foreign Secretary William Hague has said he does not want to resort to military intervention .
"This is reminiscent of Bosnia in the early nineties so I don't think we should think about it in terms of the Libya situation last year," he said.
"All our efforts are going into supporting a peaceful transition in Syria and a peaceful solution, because any violent solution would clearly involve many more deaths and a great deal more hardship for the Syrian people."