The United States said Iran is boosting 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 US Secretary of State Hillary Clinton came as US Vice President Joe Biden prepared to discuss the crisis with Russian Foreign Minister Sergei Lavrov and Syrian opposition chief Moaz al-Khatib.
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.
But media speculated that the alleged air strike on the convoy could spark a chain reaction, and reported troops on high alert in northern Israel.
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.
Israel has also raised the alarm over long-range Scud missiles, anti-aircraft systems and surface-to-surface missiles being transferred to Hezbollah.
Residents of eastern Lebanon reported soldiers and unidentified gunmen engaged in a fierce firefight near the border with Syria on Friday.
After Wednesday's alleged air strike, the foreign ministry in Damascus affirmed "Syria's right to defend itself and its territory and sovereignty."
Damascus's ambassador to Lebanon, Ali Abdel Karim Ali, said 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 told Lebanese website Al-Ahad, which is close to Hezbollah.
Damascus ally Iran was 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.
UN chief Ban Ki-moon called on all parties to "prevent tensions or their escalation in the region."
Clinton's grim assessment 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."
They "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."
She noted Russia was propping up the 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.
"What we would like to see from other countries, including Russia, is an acknowledgment that Bashar al-Assad must go and that there needs to be a transition within Syria to a new government," said Ben Rhodes, a White House national security adviser.
The US administration has resisted calls for it to deploy military assets to help Syria's rebels and has stopped short of arming them.
But it has provided non-lethal logistics, and medical and humanitarian support to rebels.
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.
On the ground, southern Damascus saw fresh clashes 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.
At least 163 people were killed nationwide on Thursday, the watchdog said.