Israeli missiles struck a research center near the Syrian capital Damascus, setting off explosions and causing casualties, Syria's state news agency reported early Sunday, citing initial reports.

If confirmed, it would be the second Israeli strike on targets in Syria in three days, signaling a sharp escalation of Israel's involvement in Syria's bloody civil war.

There was no immediate Israeli comment. However, Israel has said it will not allow sophisticated weapons to flow from Syria to the Lebanese Hezbollah militia, an ally of Syrian President Bashar al-Assad and a heavily armed foe of the Jewish state.

Two previous Israeli airstrikes, one in January and one on Friday, targeted weapons shipments apparently bound for Hezbollah, Israeli and U.S. officials have said.

The Syrian state news agency SANA reported early Sunday that explosions went off at the Jamraya research center near Damascus, causing casualties. "Initial reports point to these explosions being a result of Israeli missiles that targeted the research center in Jamraya," SANA said.

A Syrian activist group, the Britain-based Syrian Observatory for Human Rights, also reported large explosions in the area of Jamraya, a military and scientific research facility northwest of Damascus, about 15 kilometres from the Lebanese border.

An amateur video said to be shot early Sunday in the Damascus area showed a huge ball of fire lighting up the night sky. The video appeared genuine and corresponded to other Associated Press reporting.

Israel's first airstrike in Syria, in January, also struck Jamraya.

At the time, a U.S. official said Israel targeted trucks next to the research center that carried SA-17 anti-aircraft missiles. The strikes hit both the trucks and the research facility, the official said. The Syrian military didn't confirm a hit on a weapons shipment at the time, saying only that Israeli warplanes bombed the research center.

On Saturday, Israeli officials confirmed that a day earlier, Israeli aircraft targeted advanced surface-to-surface missiles in Syria that were apparently bound for Hezbollah.

The missiles were believed to be m600s, a Syrian version of Iran's Fatah 110 missile, an extremely accurate guided missile capable of travelling roughly 300 kilometres with a half-tonne warhead, an Israeli official said

The Israeli officials spoke anonymously because they had not been given permission to speak publicly about the matter.

In Washington, a Pentagon spokeswoman said she had no information relating to Sunday's report of a new Israeli airstrike in Syria.

U.S. President Barack Obama said Saturday, before the latest incident, that it was up to Israel to confirm or deny any strikes, but that the U.S. coordinates very closely with Israel.

"The Israelis, justifiably, have to guard against the transfer of advanced weaponry to terrorist organizations like Hezbollah," Obama told the Spanish-language TV station Telemundo.

U.S., Israel draw 'red line'

Israel has said it wants to stay out of the brutal Syria war, but could inadvertently be drawn in as it tries to bolster its deterrence and prevent sophisticated weapons from reaching Hezbollah.

Since the conflict in Syria erupted more than two years ago, fighting has repeatedly spilled into Turkey, Lebanon, Iraq, Jordan and the Israeli-annexed Golan Heights.

Renewed concern about a wider conflagration come as Washington considers how to respond to indications that the Syrian regime may have used chemical weapons in its civil war.

Obama has described the use of such weapons as a "red line," and the administration is weighing its options — including possible military action.

Israeli Prime Minister Benjamin Netanyahu, meanwhile, has drawn his own red line, saying repeatedly that the Jewish state would be prepared to take military action to prevent Hezbollah from obtaining new weapons that could upset the balance of power.

Israel believes Hezbollah has restocked its arsenal with tens of thousands of rockets and missiles since a 2006 war between the two ended in a stalemate.

Israel is concerned that Hezbollah will take advantage of the chaos in Syria to smuggle advanced weapons into Lebanon.

These include anti-aircraft missiles, which could hamper Israel's ability to operate in Lebanese skies, and advanced Yakhont missiles that are used to attack naval ships from the coast.

Mounting tensions

In recent days, there were signs of mounting tensions between Israel and Hezbollah.

In a warning to Israel earlier this week, Hezbollah chief Hassan Nasrallah said his militia "is ready and has its hand on the trigger" in the event of an Israeli attack on any targets in Lebanon.

The Israeli military, meanwhile, called up several thousand reservists earlier this week for what it called a "surprise" military exercise on its border with Lebanon.

Much now depends on the response from Hezbollah and Syria, analysts said.

Israeli officials have long feared that Assad may try to draw Israel into the civil war in hopes of diverting attention and perhaps rallying Arab support behind him.

But retaliation for Israeli airstrikes would come at a high price, said Moshe Maoz, an Israeli expert on Syria.

"[Assad] has his own problems and he knows that conflict with Israel would cause the collapse of his regime," Maoz said. "He could have done that long ago, but he knows he will fall if Israel gets involved."

Hezbollah, which is fighting alongside Assad's troops, appears to have linked its fate to the survival of the Syrian regime. Nasrallah, the Hezbollah chief, said this week that Syria's allies "will not allow Syria to fall into the hands of America or Israel."

On the other hand, Hezbollah could endanger its position as Lebanon's main political and military force if it confronts Israel, and it's not clear if the militia is willing to take that risk.

Chemical weapon concerns

The U.S. concerns have focused on Syria's chemical weapons.

Obama has said the use of chemical weapons would have "enormous consequences," but has also said he needs more definitive proof before making a decision about how to respond.

Obama said Friday that he didn't foresee a scenario in which the U.S. would send troops to Syria. Instead, Defense Secretary Chuck Hagel has said Washington is reviewing its opposition to arming the opposition.

The U.S. so far has balked at sending weapons to the rebels, fearing the arms could end up in the hands of al-Qaeda-linked groups or other extremists in the opposition ranks.

Secretary of State John Kerry, meanwhile, is heading to Moscow next week to try to persuade Russian President Vladimir Putin to support, or at least not veto, a fresh effort to impose UN penalties on Syria if Assad doesn't begin political transition talks with the opposition.

Russia, alongside China, has blocked U.S.-led efforts three times at the United Nations to pressure Assad into stepping down.