Russia warned on Thursday that any air strike against its ally Syria would be "unacceptable", as Israel maintained a strict silence on claims that it had bombed Syrian targets.

Russia's foreign ministry said it was "deeply concerned" after Damascus claimed a military research centre had come under Israeli fighter jet attack at dawn on Wednesday.

"If this information is confirmed, then we are dealing with unprovoked strikes against targets located on the territory of a sovereign state, which brazenly infringes on the UN Charter and is unacceptable, no matter the motive used for its justification," said a ministry statement issued in Moscow.

The strident Russian statement came after the Syrian army accused Israel of launching a strike on its military research centre in Jamraya, near Damascus.

"Israeli fighter jets violated our airspace... and carried out a direct strike on a scientific research centre in charge of raising our level of resistance and self-defence," the general command said.

The warplanes entered Syria's airspace at low altitude and under the radar, the army said, adding that two site workers were killed.

"They... carried out an act of aggression, bombarding the site, causing large-scale material damage and destroying the building," state television quoted the military as saying.

Residents told AFP that six rockets hit the complex, leaving it partially destroyed, causing a fire and killing two people.

The army, meanwhile, denied separate reports from security sources that an Israeli air strike had targeted a weapons convoy from Syria near the border with Lebanon.

Israel has frequently warned that if Syria's stockpile of chemical weapons fell into the hands of Hezbollah, a close ally of President Bashar al-Assad's regime, it would be a casus belli.

But it has also raised the alarm over long-range Scud missiles or other advanced weaponry, such as anti-aircraft systems and surface-to-surface missiles, being transferred to the Lebanese militia, against which it fought a devastating war in 2006.

Israeli officials and the military on Thursday refused to confirm or deny any involvement in the alleged attack and had no comment on the reports that its warplanes had struck a weapons convoy along the Syria-Lebanon border.

Commentators said the modus operandi was very similar to a 2007 bombing raid on an undeclared Syrian nuclear facility at Al-Kibar, widely understood to be an Israeli strike but one which was never acknowledged by the Jewish state.

The United States, which is currently hosting Israeli military intelligence chief Aviv Kochavi, also declined to comment.

-- Israeli raid 'unmasks' true cause --

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Hezbollah, in a statement issued from its Beirut headquarters, said the air strike on Syria has "unmasked" the true origins of the bloody conflict in Syria.

The Shiite militant group, an archfoe of Israel, said the attack "fully unmasked what has been happening in Syria over the past two years and the criminal objectives of destroying this country and weakening its army."

The attack took place just days after Israel moved two batteries of its vaunted Iron Dome missile defence system to the north and at a time of rising fears that the Syria conflict could see chemical weapons leaking into Lebanon.

On the political front, Syria's main opposition group was to meet Thursday in Cairo, a day after a surprise statement from its chief that he was willing to hold talks with regime officials, a Syrian National Coalition member said.

"This meeting was organised well before the Syrian National Coalition leader, Moaz al-Khatib, made his statement," SNC member Samir Nashar told AFP.

Khatib announced on Facebook Wednesday that he was "ready for direct discussions with representatives of the Syrian regime in Cairo, Tunis or Istanbul".

He laid down as conditions the release of "160,000 detainees" and that the passports of exiled citizens be renewed in embassies abroad.

On the battlefront on Thursday, fierce clashes raged between soldiers and rebel fighters on the southern outskirts of Damascus as army tanks pounded the area, a watchdog group said.

The latest fighting came a day after more than 100 people were killed in violence across Syria, according to the Observatory. The United Nations says a total of more than 60,000 people have died in the country's 22-month conflict.

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