Syria's government said on Wednesday it has "cleansed terrorists" from Al-Haffe, a Sunni enclave feared to be the site of a new massacre, as Russia accused the United States of arming rebels.
The rebel Free Syrian Army said its fighters pulled out of Al-Haffe in a tactical move to spare civilians of the beleaguered northwestern town after an eight-day regime bombardment.
As the conflict spiralled to vicious new heights, Turkey reported that 2,500 Syrians had fled across its border in 48 hours, saying numbers had increased amid attacks even targeting UN observers.
Russian Foreign Minister Sergei Lavrov accused Washington of giving arms to the rebels after his US counterpart Hillary Clinton charged that Moscow was supplying President Bashar al-Assad's regime with attack helicopters.
On the ground, at least 15 people were killed as troops and rebels clashed across the country, said the Syrian Observatory for Human Rights.
State media said government forces overran Al-Haffe, a day after UN observers came under fire trying to reach the town after the UN and opposition activists expressed fears of a massacre.
"Security and calm were restored in the area of Al-Haffe which was cleansed after armed terrorist groups assaulted citizens and vandalised and burned a number of public and private properties," SANA said.
"The authorities pursued the remaining terrorists in the villages surrounding Al-Haffe" where they "killed and arrested a number of them," the news agency said, adding that regime forces also suffered casualties.
The rebels said they "withdrew from Al-Haffe and the entire region at dawn in order to spare the lives of residents undergoing extremely violent shelling."
"The town and villages of Al-Haffe were subjected to aerial, tank and rocket bombardment, as well as a suffocating siege by regime forces and thugs," they said, adding the retreat was "to avoid falling into the regime's civil war trap."
Expressing surprise at an assessment by the UN peacekeeping chief that a sharp escalation in violence had changed the nature of the 15-month conflict, Syria said UN officials should remain "neutral, objective and precise".
"Talk of civil war in Syria is not consistent with reality ... What is happening in Syria is a war against armed terrorist groups," the foreign ministry said.
It urged regional and international powers to "stop any military or financial support for terrorist groups" operating in Syria, and urged the UN to take a "decisive stand against the crimes committed by armed groups."
The Syrian government, which is dominated by Assad's Alawite offshoot of Shiite Islam, has refused to acknowledge an uprising that erupted in March 2011 inspired by the Arab Spring.
It consistently refers to the Free Syrian Army and other armed groups as "terrorists" and has accused Washington and its regional allies of complicity in their operations.
Asked on Tuesday whether he believed Syria is in a civil war, UN peacekeeping chief Herve Ladsous said: "Yes I think we can say that. I think there is a massive increase in the level of violence, so massive indeed that in a way it indicates some change of nature."
His comments marked the first time a UN official had openly spoken of civil war in Syria.
The almost 300 observers in the United Nations Supervision Mission in Syria, or UNSMIS, face a "grave security risk," Ladsous said.
The monitors came under fire near Al-Haffe on Tuesday as they tried to reach the town just 16 kilometres (10 miles) from Assad's hometown Qardaha.
In response to US charges, Russia's Rosoboronexport arms export agency said it "does not supply weapons and military technology in contradiction with UN Security Council security requirements and other international agreements."
It "did not comment" when asked about Clinton's specific accusation of a recent Russian helicopter shipment to its Soviet-era allies in Damascus, RIA Novosti news agency reported.
Lavrov hit back on Wednesday, saying Moscow was supplying "anti-air defence systems" to Damascus in a deal that "in no way violates international laws.
"That contrasts with what the United States is doing with the opposition, which is providing arms to the Syrian opposition which are being used against the Syrian government," he said on a visit to Tehran.
Clinton had told a think-tank discussion in Washington: "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."
Stepping up the pressure on the Assad regime, French Foreign Minister Laurent Fabius said he would contact allies to draw up tougher sanctions against his top brass, as he too labelled the conflict a "civil war."
He added that France plans to ask the UN Security Council to make UN-Arab envoy Kofi Annan's peace plan "obligatory" under the UN Charter's Chapter VII, which allows measures to be imposed on countries under penalty of sanctions or the use of force.