“We can confirm… that regime jets and helicopters did fire rockets into northern Lebanon, impacting Wadi al-Khayl, near the border town of Arsal,” State Department spokeswoman Victoria Nuland told reporters.
“This constitutes a significant escalation in the violations of Lebanese sovereignty that the Syrian regime has been guilty of. These kinds of violations of sovereignty are absolutely unacceptable.”
She reminded Damascus that UN resolutions called for the strict respect of the sovereignty and territory of Lebanon.
Lebanon has publicly committed itself to staying neutral in the violence engulfing Syria, but the conflict has already exacerbated tensions and there are growing fears it could spill over into the country.
US ambassador to Lebanon, Maura Connelly, has been in contact with the Lebanese government, Nuland added.
A high-ranking Lebanese army official told AFP four missiles had been fired by Syrian warplanes in a mountainous, desert area, which observers say is ideal for the smuggling of arms and the flow of fighters across the border.