Syrian President Bashar al-Assad's fighter jets struck targets near the town of Arsal, Lebanon on Monday, according to Lebanon's state-run National News Agency.
The two countries share a porous border.
U.S. State Department spokesperson Victoria Nuland confirmed that Syrian warplanes and helicopters fired rockets into northern Lebanon, striking near Arsal.
"This constitutes a significant escalation in the violations of Lebanese sovereignty that the Syrian regime has been guilty of," Nuland said. "These kinds of violations of sovereignty are absolutely unacceptable."