WASHINGTON — President Bashar al-Assad’s forces have resorted to firing ballistic missiles at rebel fighters inside Syria, Obama administration officials said Wednesday, escalating a nearly two-year-old civil war as the government struggles to slow the momentum of a gaining insurgency.

Administration officials said that over the last week, Assad forces for the first time had fired at least six Soviet-designed Scud missiles in the latest bid to push back rebels who have consistently chipped away at the government’s military superiority.

In a conflict that has already killed more than 40,000 Syrians, the government has been forced to augment its reliance on troops with artillery, then air power and now missiles as the rebels have taken over military bases and closed in on the capital, Damascus. The escalation has not changed Washington’s decision to avoid military intervention in Syria — as long as chemical weapons are not used — but it did prompt a rebuke.

“As the regime becomes more and more desperate, we see it resorting to increased lethality and more vicious weapons moving forward, and we have in recent days seen missiles deployed,” said Victoria Nuland, a State Department spokeswoman.