The Syrian army has confirmed that Israeli jets crossed into Syria below radar level at dawn and carried out airstrikes against military targets.
It is unclear, however, what the exact nature of the target of Wednesday's air strikes was. Syrian state media said that the strikes targeted a military research centre near Damascus, while regional security sources told the Reuters and AP news agencies that the target was a convoy of trucks allegedly carrying anti-aircraft missiles for Hezbollah in Lebanon.
"Israeli fighter jets violated our airspace at dawn today and carried out a direct strike on a scientific research centre in charge of raising our level of resistance and self-defence," the army's general command said in a statement carried by state news agency SANA on Wednesday evening.
The strike came "after terrorist groups made several failed attempts in the past months to take control of the site," the statement added, using the term President Bashar al-Assad's government uses for rebel fighters.
The jets entered the Syrian airspace via Mount Hermon, or Jabal el-Sheikh in Arabic, the army said.
"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.
The army added that two site workers were killed in the strike, and five others wounded.
The Syrian army and Hezbollah both denied earlier reports that the overnight air strike had targeted a weapons convoy from Syria to the armed group in Lebanon.
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News agencies quoted US and regional officials as saying that Israel had conducted an air strike inside Syria near the border with Lebanon, hitting a convoy of trucks.
"The target was a truck loaded with weapons, heading from Syria to Lebanon," said one Western diplomat on Wednesday.
A source among rebels fighting Assad said an air strike around dawn (04:30 GMT) on Wednesday blasted a convoy on a mountain track about five kilometres south of where the main Damascus-Beirut highway crosses the border.
The regional officials said Israel had been planning in the days leading up to the air strike to hit a shipment of weapons bound for Hezbollah.
They said the shipment included sophisticated Russian-made SA-17 anti-aircraft missiles, which would be strategically "game-changing" in the hands of Hezbollah.
All officials spoke on condition of anonymity because they were not authorised to discuss the strike.
In a statement released on Thursday, Hezbollah denied the report, terming the Israeli strikes a "raid of aggression" and criticising the international community for not speaking out against it.
Russia 'deeply concerned'
Israel and the United States declined to comment on the strikes. Russia on Thursday said that it found the reports of the strikes of great concern.
"If this information is confirmed, then we are dealing with unprovoked attacks on targets on the territory of a sovereign
country, which blatantly violates the UN charter and is unacceptable, no matter the motives to justify it," the Russian
foreign ministry said in a statement.
Among Israeli security officials' chief fears is that Hezbollah could get Syrian chemical arms and SA-17 anti-aircraft missiles.
If that were to happen, it would change the balance of power in the region and greatly hinder Israel's ability to conduct air violations in Lebanon.
The military in Lebanon, which shares borders with both Israel and Syria, said on Wednesday that Israeli warplanes had sharply increased their activity over Lebanon in the past week, including at least 12 sorties in less than 24 hours in the country's south.
Israeli violations of Lebanese airspace occur on a daily basis and Lebanese authorities routinely lodge complaints at the UN against the flights.