Russia is sending attack helicopters to Syrian President Bashar Assad's regime, the US claimed tonight.
And Washington warned that the Arab country's 15-month siege could become even deadlier, claiming that the country has gone into "civil war".
US Secretary of State Hillary Clinton said at a conference today:
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.
Mrs Clinton's comments at a public appearance with Israeli President Shimon Peres in Washington augured poorly for a peaceful solution to Syria's conflict.
ITV News' US Correspondent Robert Moore reported that the conflict is creating diplomatic tensions.
Earlier today, United Nations peacekeeping chief Hervé Ladsous declared that Syria is now in a civil war. He also said that the Assad regime has lost "large chunks of territory" in some cities.
A spokesman said later on that 200 Free Syrian Army rebels have withdrawn from the besieged Sunni Muslim town of Haffeh under pressure from bombardment by forces loyal to the president.
And it's not only rebel forces facing the pressure. Angry crowds blocked UN observers from reaching an embattled rebel-held town, hurling stones and metal rods at the monitors' vehicles as civilians became increasingly frustrated with the situation.
Diplomatic hopes have rested on Washington and Moscow agreeing on a transition plan that would end the four-decade-long Assad regime.
With diplomacy at a standstill, the reported shipment of helicopters suggests a dangerous new turn for Syria after more than a year of harsh government crackdowns on mainly peaceful protests and the emergence of an increasingly organised armed insurgency.