Syria summoned the head of a United Nations mission in the Golan Heights on Thursday to protest against an Israeli air raid which Damascus said is a violation of a disengagement accord that followed the last major war between the two countries.
State media said Major General Iqbal Singha, head of the UNDOF peacekeeping force, was summoned to the Foreign Ministry where the protest was delivered.
Meanwhile, Syria warns of a possible "surprise" response to Israel's attack on its territory as Russia condemned the air strike as an unprovoked violation of international law.
Damascus could take "a surprise decision to respond to the aggression of the Israeli warplanes", Syrian ambassador to Lebanon Ali Abdul-Karim Ali said a day after Israel struck against Syria.
"Syria is engaged in defending its sovereignty and its land," Ali told a website of the Lebanese militant group Hezbollah.
Syria and Israel have fought several wars and in 2007 Israeli jets bombed a suspected Syrian nuclear site, without a military response from Damascus.
UN leader Ban Ki-moon is gravely concerned about the reports of an Israeli air strike on Syria but cannot independently verify what happened, a spokesman said Thursday.
"The secretary general calls on all concerned to prevent tensions or their escalation in the region." the deputy UN spokesman Eduardo del Buey said.
Israel has remained silent on the attack and there has been little reaction from its Western backers, but Syria's allies in Moscow and Tehran were quick to denounce the strike.
Russia 'deeply concerned'
Russia, which has blocked Western efforts to put pressure on Syria at the United Nations, said that any Israeli air strike would amount to unacceptable military interference.
"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 U.N. Charter and is unacceptable, no matter the motives to justify it," the Foreign Ministry said in a statement.
Details of Wednesday's strike remain sketchy and, in parts, contradictory. Syria said Israeli warplanes, flying low to avoid detection by radar, crossed into its airspace from Lebanon and struck the Jamraya military research centre.
The aggression resulted in serious material damage and killed two workers and injured five others.
But the diplomats and rebels said the jets hit a weapons convoy heading from Syria to Lebanon, apparently destined for Assad's ally Hezbollah, and the rebels said they - not Israel - hit Jamraya with mortars.