Syria has made an official complaint to the United Nations over an alleged Israeli airstrike on the border it shares with Lebanon.
A US official told the BBC the attack targeted a convoy of trucks carrying Russian-made missiles to militant group Hezbollah in Lebanon something Syria denies.
Instead it claims a scientific research centre in Jamraya, to the north of the country, was hit, killing two people and wounding five.
According to Syrian state media the foreign ministry summoned the UN commander in the Golan to make a formal protest, saying Israel's action breached a 1974 agreement.
Both Syria and Iran have threatened to retaliate, with Syrian Ambassador to Lebanon, Ali Abdul-Karim Ali telling the Associated Press that Damascus "has the option and the capacity to surprise in retaliation."
Iran has promised "repercussions," while Russia labelled the strike "unacceptable." However Israel has refused to confirm or comment on the incident.
Escalating tensions in the region come a day after the UN's envoy said Syria's war had descended into "unprecedented levels of horror" and that only the international community that can help the beleaguered nation.
A man reads the Qu'ran next to the bodies of Syrian civilians
Lakhdar Brahimi told the divided UN Security Council it must act in the wake of the massacre of at least 71 young men and at least one boy as young as 12, with bodies found dumped in a river in Aleppo, after being executed by Bashar al Assad's forces.
The Syrian government has denied responsibility and says the men were executed by an Islamist faction. State news agency Sana said the perpetrators were the jihadists Al Nusra Front.
Brahimi said: "Syria is breaking up before everyone's eyes. Only the international community can help, and first and foremost the Security Council."
Lakhdar Brahimi, the UN's envoy to Syria
He warned that the Assad regime was "irreparably damaged" but said it would be hard to shift the regime from power, and both sides were guilty of "atrocious crimes."
In a gruesome video posted online, 51 muddied male bodies can be seen lying along what appears to be the Queiq River in Aleppo's rebel-held Bustan al-Qasr neighbourhood.
Human rights organisations and the UN estimates that more than 60,000 people have been killed in the conflict.
The Britain-based Syrian Observatory for Human Rights, a Syrian opposition group, said the Aleppo victims were found with their hands bound and a single, execution-style, bullet wound to the head.
On Monday, the UN warned it would not be able to help millions of Syrians affected by the fighting without more money and will appeal for aid at a conference in Kuwait this week.