Syrian warplanes have bombed the border area with Lebanon for the first time, Lebanese army officials say, reportedly targeting Syrian rebel positions inside Lebanon.
Officials on Monday said that four missiles hit the Lebanese border town of Arsal, where many residents back the uprising against Syrian President Bashar al-Assad.
The mountainous area is also ideal for the smuggling of arms and the flow of fighters across the border.
Local residents say the missiles had fallen in an agricultural area and there were no injuries in the incident.
Al-Manar television, which belongs to Hezbollah, a pro-Assad Lebanese Shia movement, reported that the warplanes had targeted two barns used by "armed men" in the Wadi al-Khayl area of Arsal.
The US State Department confirmed that Syrian government aircraft fired rockets into Lebanon.
"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," State Department spokeswoman Victoria Nuland told reporters at her daily briefing.
France condemned the raid, saying Assad's forces had seriously violated the sovereignty of its neighbour.
"The air raid carried out by the armed forces of the Syrian regime in Lebanese territory, in the Arsal area, constitutes a
new and serious violation of Lebanon's sovereignty," foreign ministry spokesman Philippe Lalliot said in a statement
The attack comes four days after Damascus warned it may strike at Syrian rebels taking refuge across the frontier.
Lebanon has a policy of "dissociation" from the two-year civil war in Syria but officials say they feel their country is increasingly at risk of being dragged into a conflict that the UN says has killed more than 70,000 Syrians.
Syria's Foreign Ministry said on Thursday that a "large number" of opposition fighters had crossed Lebanon's northern border into the Syrian town of Talkalakh last week.
"Syria expects the Lebanese side to prevent these armed terrorist groups from using the borders as a crossing point, because they target Syrian people and are violating Syrian sovereignty," it said in a message to the Lebanese government, according to state media.
It said Syria's "patience is not unlimited", even though "Syrian forces have so far exercised restraint from striking at armed gangs inside Lebanese territory."
Lebanon’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.