The head of UN peace keeping has said that situation in Syria has developed into a "civil war."
Speaking to reporters in New York, Herve Ladsous said that the Syrian government has lost control of "large chunks" of territories and is now trying to retake them.
His comments came as UN monitors reported that they were fired at and prevented from entering Haffa province in northern Syria, where there are reports of regime forces shelling the village, currently held by opposition fighters.
An angry crowd threw stones at the UN monitors as they turned back. They were fired at as they left, but none of the UN workers were hurt, reports the BBC.
Washington has expressed alarm that President Bashar al Assad's regime may be stepping up attacks on rebel strongholds in the region, in the wake of atrocities reported in villages in the Hama province, which followed the killing of 108 people in Houla earlier in May, many of them reported to be women and children
The reports came as the US Secretary of State Hillary Clinton expressed concerns that Russia was sending attack helicopters to support Syrian regime forces.
She told a press conference that Russian arms could "escalate the conflict quite dramatically." Russian assurances that arms shipments were unrelated with Syrian regime were "patently untrue" she added.
Ladsous raised fears earlier over reports of attack helicopters. He told reporters: "Now we have confirmed reports of not only of the use of tanks and artillery but also attack helicopters,"
"This is really becoming large scale" he added, reported the Guardian.
His comments came as as United Nations report stated that children as young as nine have been used as human shields, sexually assaulted and tortured by forces loyal to Syrian President Bashar al-Assad.
Its annual report on children and armed conflict states that government forces, including the Syrian Armed Forces, the intelligence forces and the Shabbiha militia have all targeted children.
Some have been tortured for information about opposition forces and others have been placed on tanks and buses to prevent rebel groups from opening fire.
The report said: “Children as young as nine years of age were victims of killing and maiming, arbitrary arrest, detention, torture and ill-treatment, including sexual violence, and use as human shields.”
The chilling document also describes how former members of the Syrian Armed Forces admitted that on occasions they have been ordered to fire arbitrarily into unarmed groups of civilian protesters, with their commander knowing they contained women and children.
It said: “[One member], during protests in Tall Kalakh in December 2011, was given an order by his commander to shoot without distinction.
“During the armed break-up of the demonstrations, the witness saw three girls between approximately 10 and 13 years of age who had been killed by the Syrian Armed Forces.
“In another similar incident in Aleppo in the fourth quarter of 2011, a former member of the intelligence forces witnessed the killing of five children in a secondary school during demonstrations.”
UN special representative Radhika Coomaraswamy told the BBC that in her experience she had never known a conflict where children were specifically targeted.
However, she also criticised the Free Syrian Army for putting children at risk by using them “in medical and service orientated jobs” on the front line.
Meanwhile, shelling on residential areas in Homs continued on Monday, according to the Activists News Association.
The group, which aims to support "citizen journalists" posted a video of the shelling on YouTube, which can be seen below - although there is no way of confirming its authenticity.