Syrian government forces have fired Scud missiles at rebels in recent days, according to a US official, a move which escalates the two-year-old conflict.
The official confirmed a report that forces of president Bashar Assad have fired the missiles from the Damascus area into northern Syria.
He estimated that the number of Scuds fired was more than half a dozen.
The official said there was no indication that chemical weapons were on board the missiles.
Officials have said over the past week that they feared rebel advances were prompting Mr Assad to consider using chemical weapons.