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The United States said Iran is stepping up its support for the Syrian regime and that Russia is still arming it, heightening fears on Friday that the conflict may spill over the country's borders.
The assessment by Secretary of State Hillary Clinton came as US Vice President Joe Biden prepared to discuss the crisis at meetings with Russian Foreign Minister Sergei Lavrov, Syrian opposition chief Moaz al-Khatib and UN-Arab League envoy Lakhdar Brahimi.
Fresh concerns about the 22-month conflict drawing in the wider region arose after Damascus threatened to retaliate over a reported Israeli air raid and key ally Iran warned the attack would have "grave consequences."
President Bashar al-Assad's regime accused Israel of sending its warplanes to attack a military research centre in Jamraya, near Damascus, on Wednesday.
Israel has so far maintained a stony silence, as well as over separate reports its aircraft had hit a weapons convoy near the Lebanese border.
Israel has frequently warned that if Syria's chemical weapons fall into the hands of Lebanon's Shiite movement Hezbollah, Israel's arch-foe and close Damascus ally, this would be a casus belli.
The Lebanese army, meanwhile, said two soldiers were killed in a clash with unidentified gunmen in a village near the border with Syria on Friday as the military hunted a man wanted for "several terrorist attacks".
After Wednesday's alleged air strike, Damascus affirmed "Syria's right to defend itself and its territory and sovereignty."
Its ambassador to Lebanon, Ali Abdel Karim Ali, said Syria "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 told Lebanese website Al-Ahad, which is close to Hezbollah.
Damascus ally Iran was also strident, with Deputy Foreign Minister Hossein Amir Abdollahian warning that the "Zionist regime's attack on the outskirts of Damascus will have grave consequences for Tel Aviv," ISNA news agency reported.
Russia expressed "deep concern" over the reported strike, saying it would be a brazen infringement of the UN charter and unacceptable.
-- Dangers of proxy war --
UN chief Ban Ki-moon called on all parties to "prevent tensions or their escalation in the region," as Clinton warned 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.
"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."
She also noted Russia was propping up the Damascus regime despite US efforts to work for an international solution to a conflict the UN says has cost more than 60,000 lives.
"We have reason to believe that the Russians continue to supply financial and military assistance in the form of equipment to Assad."
Biden is to meet Lavrov and Khatib on Saturday on the sidelines of the Munich Security Conference. Khatib was to hold late-night talks on Friday with Brahimi.
In Geneva, the UN's children's agency said some 420,000 people -- half of them children -- in the central region of Homs desperately need humanitarian aid.
And head of the International Committee of the Red Cross, Peter Maurer, urged all sides of the Syrian conflict to help his organisation access more of the war-ravaged country with desperately needed aid, in an interview with AFP.
The UN agency for Palestinian refugees said the conflict in Syria has displaced half of the country's 500,000 Palestinian refugees.
On the ground, clashes between soldiers and rebels broke out in southern Damascus on Friday, while army shelling hit a town in northern Aleppo province and Homs city was also pounded, the Syrian Observatory for Human Rights said.
The monitoring group, which relies on a network of activists and medics on the ground, said at least least 80 people, 32 of them soldiers, were killed in violence across the country.
In the weekly anti-regime protests, demonstrators took to the streets on Friday under the slogan: "The international community is an accomplice of Assad's crimes."
"We will bring Assad to justice no matter what lives it takes, no matter how much catastrophe it makes," one banner read at a town in the Idlib region of northwest Syria.
Fifty-three people were killed in a January 24 suicide car bombing at a Syrian military intelligence headquarters in Damascus province carried out by Islamist rebel fighters, according to the Observatory.