Syrian forces have fired Scud missiles at rebel groups in the northern part of the country over the last few days, according to senior U.S. officials.

The use of the missiles marks a potentially significant expansion of the civil war, which has already killed more than 40,000 people. It comes as the Syrian rebels have been gaining momentum in their fight to oust Syrian President Bashar al-Assad and his regime.

While the White House hasn’t yet confirmed the reports, it did come up during today’s daily press briefing. “If true, this would be the last desperate act from a regime that has shown utter disregard for innocent life, utter disregard for the lives of its own citizens,” said White House press secretary Jay Carney. An unidentified official also called the move “desperate”, noting the increased likelihood for civilian casualties with such weapons, and said their use could be the latest indication that Syrian President Bashar al-Assad is afraid of the opposition forces.