US officials said Israel launched a rare airstrike inside Syria on Wednesday.
The target was a convoy believed to be carrying anti-aircraft weapons bound for Hezbollah, the powerful Lebanese militant group allied with Syria and Iran.
In Israel, a lawmaker close to hard-line Prime Minister Benjamin Netanyahu stopped short of confirming involvement in the strike. But he hinted that Israel could carry out similar missions in the future.
The attack has inflamed regional tensions already running high over Syria's 22-month-old civil war.
Israeli leaders in the days leading up to the airstrike had been publicly expressing concern that Syrian President Bashar Assad may be losing his grip on the country and its arsenal of conventional and nonconventional weapons.
Regional security officials said Wednesday that the targeted shipment included sophisticated Russian-made SA-17 anti-aircraft missiles, which if acquired by Hezbollah would enhance its military capabilities by enabling the militants to shoot down Israeli jets, helicopters and surveillance drones.
The officials spoke on condition of anonymity because they were not authorized to brief the media.
The Syrian military denied there was any weapons convoy and said low-flying Israeli jets had crossed into their country over the Israeli-occupied Golan Heights to bomb a scientific research center near Damascus.
It said the target was in the area of Jamraya, northwest of Damascus and about 15 kilometers (10 miles) from the Lebanese border.
Maj. Gen. Abdul-Aziz Jassem al-Shallal, who in December became one of the most senior Syrian army officers to defect, told The Associated Press by telephone from Turkey that the site they said was targeted is a "major and well-known" center to develop weapons known as the Scientific Research Center.
Al-Shallal, who until his defection was commander of the military police, said no chemical or nonconventional weapons are at the site. He added that foreign experts, including Russians and Iranians, are usually present at such centers.
Syrian Ambassador to Lebanon Ali Abdul-Karim Ali threatened retribution for the Israeli airstrike, saying Damascus "has the option and the capacity to surprise in retaliation."
He told Hezbollah's al-Ahd news website that it was up to the relevant authorities to prepare the retaliation and choose the time and place.
The Syrian foreign ministry summoned Major-General Iqbal Singh Singha, the head of mission and force commander for United Nations Disengagement Observer Force on the Golan Heights, to complain about the Israeli violation.
The force was established in 1974 following the disengagement of Israeli and Syrian forces in the area and has remained there since to maintain the cease-fire.
Israel captured the Golan, a strategic plateau, from Syria in the 1967 Middle East war.
Hezbollah condemned the attack as "barbaric aggression" and said it "expresses full solidarity with Syria's command, army and people."
The group did not mention any weapons convoy in the statement but said the strike aimed to prevent Arab and Muslim forces from developing their military capabilities.
In Iran, the semi-official Fars news agency quoted deputy foreign minister Hossein Amir Abdollahian as saying the raid will have significant implications for Israel.
Iranian foreign minister Ali Akbar Salehi condemned the airstrike on state television, calling it a clear violation of Syrian sovereignty. Iran is Syria's strongest ally in the Middle East, and has provided Assad's government with military and political backing for years.
Russia, Syria's most important international ally, said this appeared to be an unprovoked attack on a sovereign nation. Moscow said it was taking urgent measures to clarify the situation in all its details.
"If this information is confirmed, we have a case of unprovoked attacks on targets in the territory of a sovereign state, which grossly violates the UN Charter and is unacceptable," Russia's Foreign Ministry said in a statement. "Whatever the motives, this is not justified."
Israeli lawmaker Tzachi Hanegbi, who is close to Prime Minister Netanyahu, said pinpoint strikes are not enough to counter the threat of Hezbollah obtaining sophisticated weaponry from Syria.
"Israel's preference would be if a Western entity would control these weapons systems," Hanegbi said.
"But because it appears the world is not prepared to do what was done in Libya or other places, then Israel finds itself like it has many times in the past facing a dilemma that only it knows how to respond to," he added.
He was referring to NATO's 2011 military intervention in Libya that helped oust dictator Moammar Gadhafi.
"Even if there are reports about pinpoint operations, these are not significant solutions to the threat itself because we are talking about very substantial capabilities that could reach Hezbollah," he said.
Syria's civil war has sapped Assad's power and threatens to deprive Hezbollah of a key supporter, in addition to its land corridor to Iran. The two countries provide Hezbollah with the bulk of its funding and arms.
Earlier this week, Netanyahu warned of the dangers of Syria's "deadly weapons," saying the country is "increasingly coming apart."
The same day, Israel moved a battery of its new "Iron Dome" rocket defense system to the northern city of Haifa, which was battered by Hezbollah rocket fire in the 2006 war. The Israeli army called that move "routine."
The Israeli army won't say whether Iron Dome was sent north in connection to this operation. It does note that it has deployed the system in the north before.