Syria's regime has fired at least six Scud ballistic missiles on its own people since Monday in a significant escalation that means it has used every weapon in its arsenal short of chemical weapons.
US officials said the regime had used the weapons to target "rebels hiding in playgrounds".
Sources said that intelligence reports indicated that the regime first fired a Scud missile on Monday and the firing has continued.
"We condemn this in strongest possible terms that demonstrates the appalling brutality of the regime and its desperation to go to any lengths to deny his people their legitimate aspiration," a NATO official said.
"Allied intelligence, surveillance and reconnaissance assets have detected the launch of a number of unguided, short-range ballistic missiles inside Syria this week. Trajectory and distance travelled indicate they were Scud-type missiles," the official said in Brussels.
The news emerged as 114 countries held a summit in Morroco to recognise a new Syrian opposition coalition. The US has also declared the coalition is the "legitimate representative" of the country's people.
Meanwhile, a blast went off at the Syrian interior ministry in Damascus yesterday. There was no confirmation of casualties.
The conflict started nearly 21 months ago as an uprising against Syrian President Bashar al-Assad, whose family has ruled the country for four decades.
According to activists, more than 40,000 people have been killed since March 2011.
The firing of Scud missiles is an ominous escalation in the conflict, not least because it brings the threat of chemical weapons one step closer.
Western officials have raised concerns that the increasingly desperate president might unleash his chemical weapons stockpiles against rebels, saying that to do so would be crossing a "red line".
Syria is believed to have a formidable arsenal of chemical weapons, including sarin and mustard gas, although its exact dimensions are not known. The country is not a signatory to the 1997 Convention on Chemical Weapons and thus is not obliged to permit international inspection.
The government in Damascus has been careful not to confirm it has chemical weapons, while insisting it would never use such weapons against its own people.
White House press secretary Jay Carney said he could not confirm the Scud report, but said if true it would be a sign of desperation.
"The idea that the Syrian regime would launch missiles, within its borders, at its own people, is stunning, desperate and a completely disproportionate military escalation," Mr Carney said. (© Daily Telegraph, London)