Syrian warplanes bombed the border area with Lebanon for the first time on Monday, a high-ranking Lebanese army official told AFP, reportedly targeting Syrian rebel positions inside Lebanon.
"Syrian planes bombed the border between Lebanon and Syria but I cannot yet say if they hit Lebanese territory or only Syrian territory," the military official said, speaking on condition of anonymity.
He said four missiles had been fired by Syrian warplanes.
A Lebanese security services official on the ground confirmed that four missiles were fired at Syrian rebel positions on the Lebanese side of the border.
He said the attacks targeted the town of Arsal, in east Lebanon, close to the Syrian border, where many residents back the uprising against Syrian President Bashar al-Assad.
The mountainous, desert area is also ideal for the smuggling of arms and the flow of fighters across the border.
Al-Manar television, which belongs to the pro-Syrian Lebanese Shiite Hezbollah movement, reported that the warplanes had targeted two barns used by "armed men" in the Wadi al-Khayl area of Arsal.
Local resident Suheil Fliti told AFP that the missiles had fallen in an agricultural area and there were no injuries in the incident.
Lebanon has publicly committed itself to staying neutral in the violence engulfing Syria, but the conflict has already exacerbated tensions in the country and there are growing fears it could spill over into Lebanon.
The country's opposition largely backs the Syrian uprising, while Hezbollah and its allies stand by Assad's regime.
Syrian rebels accuse Hezbollah of dispatching fighters to battle alongside Assad's force, while Damascus last week threatened to respond with force if "armed terrorist gangs" continued to infiltrate from Lebanon.
"Syrian forces are showing restraint by not striking these gangs inside Lebanese territory to prevent them crossing into Syria, but this will not go on indefinitely," the foreign ministry said in a message to its Lebanese counterpart.
On Saturday, a Lebanese government source said Beirut was taking the warning "very seriously" and that "intensive consultations are underway to find the best way to control the border."