The U.S. State Department confirmed on Monday that Syrian government aircraft fired rockets into northern Lebanon and it described the incident as "a significant escalation."

"We can confirm what you are seeing in the press, that regime jets and helicopters did fire rockets into northern Lebanon," State Department spokeswoman Victoria Nuland told reporters at her daily briefing.

"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."