Syria has threatened to retaliate over what it says was an Israeli air raid, which risks taking the conflict beyond Syria's borders, as Washington warned that Iran was stepping up support for Damascus.

Israel maintained a stony silence over Syria's claims, as well as over separate reports that its jets had hit a weapons convoy near the Lebanese border.

Syria's foreign ministry said Israel "and the states that protect it" are responsible for the air strike, and "affirms Syria's right to defend itself and its territory and sovereignty," state news agency SANA reported.

It called on "all the competent UN bodies to take the necessary steps given this grave Israeli violation, and to guarantee that it will not happen again."

UN chief Ban Ki-moon expressed "grave concern" and called on all parties to "prevent tensions or their escalation in the region."

Meanwhile US Secretary of State Hillary Clinton warned Thursday that Iran was stepping up its support for Syrian President Bashar al-Assad.

"The Iranians have made it clear for some time that keeping Assad in power was one of their highest priorities. We believe they have acted on that by sending in more personnel, not only to help Assad, but to support and advise military security forces," Clinton said in Washington.

In her last media interview as secretary of state, Clinton said "the Iranians have been actively involved from the very beginning. It appears that they may be increasing that involvement and that is a matter of concern to us."

Clinton also noted that despite US efforts to bring Moscow on board to work for an international solution to the 22-month war in Syria that has claimed some 60,000 lives, Russia was continuing to prop up the regime.

"We have reason to believe that the Russians continue to supply financial and military assistance in the form of equipment to Assad," she said

And the top US diplomat gave a grim assessment of the progression of the war, warning of "the dangers of an increasing civil war and a potential proxy war."

"The worst kind of predictions of what could happen, both internally and spilling over the borders of Syria are certainly within the realm of the possible now," she said.

Damascus's ambassador to Lebanon, Ali Abdel Karim Ali, stressed Syria's right to respond to "the Zionist aggression."

"The Israelis, and the United States behind them, along with their Arab and regional accomplices, realise that Syria, which defends its sovereignty and territory, may decide to respond by surprise to this aggression."

"It is up to the competent powers to choose the appropriate answer, and to determine the means and the place," Ali added in remarks to Lebanese website Al-Ahad, which is close to the powerful Shiite group Hezbollah.

Reaction from close Damascus ally Iran was strident.

Deputy Foreign Minister Hossein Amir Abdollahian warned, without elaborating, that the "Zionist regime's attack on the outskirts of Damascus will have grave consequences for Tel Aviv," the ISNA news agency reported.

In the past, Tehran has said any Israeli attack on Syria would be considered an attack on Iran.

Russia's foreign ministry said it was "deeply concerned" but was still trying to verify Syria's allegations.

Late on Wednesday, Syria accused Israel of launching a dawn strike on a 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 army general command said, saying two workers were killed.

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

Hezbollah denounced "a new Zionist aggression."

Meanwhile, the White House said Vice President Joe Biden will discuss Syria on Saturday with Russian Foreign Minister Sergei Lavrov and Syrian opposition chief Moaz al-Khatib.

Israel has frequently warned that if Syria's stockpile of chemical weapons fell into Hezbollah hands, this would be a casus belli.

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 Hezbollah.

Israeli officials and the military on Thursday refused to confirm or deny any involvement in the alleged attack.

Meanwhile, the main Syrian National Coalition opposition group said any talks on the country's political future must be about the departure of the Assad regime.

It also welcomed "any political solution or international effort aimed at achieving that objective."

In Brussels, EU foreign ministers discussed whether to lift an arms embargo on Syria, to help the opposition. A decision is expected in mid-February.

On the ground, the Syrian Observatory for Human Rights said at least 109 people were killed nationwide on Thursday in a conflict the UN says has left more than 60,000 dead in 22 months.