Syrian President Bashar al-Assad's forces have fired Scud-style ballistic missiles against rebels in recent days, US and NATO officials have confirmed.
A US official, speaking on condition of anonymity, confirmed the use of Scuds. US officials said they were not aware of any previous uses of the missiles.
Scuds have a range of up to a few hundred kilometres and are best-known internationally from the 1991 Gulf War, when Iraqi leader Saddam Hussein fired them at Israel.
Mr Assad's forces have in the past relied on artillery, helicopters and attack jets, all of which are much more useful in close urban combat.
However, the lightly armed rebels are increasingly obtaining better weapons to fight back, including the ability to shoot down aircraft.
Earlier today, Russia accused the US of undermining diplomatic efforts aimed at ending the conflict in Syria.
Last night, US President Barack Obama announced that the United States was to recognise the Syrian opposition coalition as the legitimate representative of the people there.
Russian Foreign Minister Sergei Lavrov said Mr Obama's decision to follow similar ones taken by the EU, Turkey and several Arab states ran against agreements to seek political transition in Syria.
He said the US appeared to be counting on a military victory against President Bashar al-Assad.
"This contradicts the agreements set in the Geneva communiqué that proposes the start of all-Syrian dialogue between the representatives named by the government on the one side and the opposition on the other," Mr Lavrov said.
"During consultations that took place three days ago in Geneva, we thought the Americans understood the necessity of creating conditions for all-Syrian dialogue to include government members too," Mr Lavrov added.
"So for us it is quite an unexpected turn and we will seek to clarify what exactly they (the United States) have in mind."
Mr Obama's announcement, made during an interview with Barbara Walters of ABC News, grants new legitimacy to the rebel group and marks a new phase in US efforts to isolate the Assad regime.
The US is following in the footsteps of Britain and the European Union, who both recognised the Syrian opposition group last month.Mr Obama said: "We've made a decision that the Syrian opposition coalition is now inclusive enough, is reflective and representative enough of the Syrian population that we consider them the legitimate representative of the Syrian people in opposition to the Assad regime."
"It's a big step," Mr Obama said of the decision.
The move does not include the provision of weapons, but it opens the door for that possibility in the future.