Syria civil war
A file photo of a man trying to extinguish a fire following shelling by forces loyal to Syria\'s President Bashar al-Assad, according to the Free Syrian Army, in Jobar area in Damascus. - Reuters

Syrian jets and helicopters have bombarded Lebanon with rockets in a "significant escalation" of the civil war in the region, while opposition groups claim to have struck President Bashar al-Assad's palace in Damascus.

The rockets are thought to have struck the Lebanese town of Arsal, causing some damage. However it is reported that the missiles struck empty buildigns, and no casualties have been reported so far.

The aerial bombardment is said to be a response to increasing cross-border incursions from Lebanese militants to assist the rebels fighting Assad's troops.

Last week, Damascus warned Lebanon about the incursions, saying Assad's forces were not attacking "concentrations of armed gangs inside Lebanese territory in order to prevent them from crossing into Syrian territory. But this will not last indefinitely".

The rebels have shut a key route in the border region as the conflict enters a new phase. "The [Free Syrian Army] made it clear to the drivers of dozens of trucks stranded in Syria that they should not cross again, and that the road would be closed for the time being," the head of Lebanon's Farmers' Association, Antonie Howayek, told the Daily Star.

The US has condemned the latest Syrian attack saying: "We can confirm ... that regime jets and helicopters did fire rockets into northern Lebanon, impacting Wadi al-Khayl, near the border town of Arsal."

Victoria Nuland, the US State Department spokesperson added: "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."

The French foreign ministry has also criticised the attack by the Syrian regime and said it is "a new serious violation of Lebanon's sovereignty".

Meanwhile the rebels fired mortar bombs at government buildings in Damascus, including the presidential palace, to mark the second anniversary of the uprising against Assad.

"The operation comes in conjunction with the second anniversary of the start of our revolution; the revolution of freedom and dignity against the despotic regime of the criminal Bashar al-Assad," said a statement on an opposition Facebook page.

The statement added that they fired, "a number of 120 mm heavy calibre mortars... in a joint operation coordinated with battalions operating in Damascus".

Although the Syrian Observatory for Human Rights said several rockets fell in the area around the palace, it could not confirm whether or not the building itself was actually struck.

The Syrian Observatory for Human Rights, which has a network of activists across the country, said several rockets fell in districts near the presidential palace. But it said it could not confirm if the building was actually hit, nor whether there were any casualties.

The rebels' claim could not be independently verified.

Earlier, the rebels chose Ghassan Hitoo as prime minister to administer the rebel-held regions in the country. Hitoo was elected by the rebel leaders in Turkey's capital Istanbul.