WASHINGTON — President Bashar 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 past 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.