Israeli warplanes struck targets in the Syrian capital Sunday for the second time in three days, officials and activists said, unleashing a series of massive explosions and raising fears of possible wider conflict in the region.

The attacks, which Israeli officials said targeted sophisticated Iranian-made guided missiles believed to be bound for Lebanon's Hezbollah militant group, marked a sharp spike in Israel's involvement in Syria's bloody civil war. Syrian state media reported that Israeli missiles struck a military and scientific research centre near Damascus and caused casualties.

Syria's government called the attacks against its territory a "flagrant violation of international law" that has made the Middle East "more dangerous." It said "Israel should know that our people and state do not accept humiliation" and warned Syria has the right "to defend its people by all available means."

The generally muted response, read out by the information minister after an emergency government meeting, appeared to signal that Damascus did not want the situation to escalate.

Instead, it tried to use the strikes to taint the opposition, claiming the attacks were evidence of an alliance between Israel and Islamist extremist groups trying to overthrow President Bashar al-Assad.

The air raids pose a dilemma for the regime: Failure to respond could make it look weak and open the door to more strikes, but retaliating militarily against Israel would risk dragging the Jewish state and its powerful army into a broader conflict.

The tempo of the new strikes added a dangerous dynamic to the conflict, fuelling concerns that events could spin out of control and spark a regional crisis.

Israel's military, possibly bracing for Syrian retaliation, deployed two batteries of its Iron Dome rocket defence system to the north of the country on Sunday. It described the move as part of "ongoing situational assessments."

A senior Israeli official, who spoke on condition of anonymity in order to disclose information about a secret military operation, confirmed that Israel launched an airstrike in the Syrian capital early Sunday but did not give more precise details about the location. The target was Fateh-110 missiles, which have precision guidance systems with better aim than anything Hezbollah is known to have in its arsenal, the official told The Associated Press.

The airstrikes come as Washington considers how to respond to indications that the Syrian regime may have used chemical weapons in its civil war. U.S. President Barack Obama has described the use of such weapons as a "red line," and the administration is weighing its options — including possible military action.

The White House declined for a second day to confirm or comment directly on the airstrikes in Syria, but said Obama believes Israel has the right to defend itself against threats from groups like Hezbollah.

"The Israelis are justifiably concerned about the threat posed by Hezbollah obtaining advanced weapons systems, including some long-range missiles" White House spokesman Josh Earnest told reporters aboard Air Force One as Obama travelled to Ohio.

Iran, a close ally of the Assad regime and Hezbollah, condemned the airstrikes, and a senior official hinted at a possible response not from Tehran but rather its proxy, Hezbollah.

Gen. Masoud Jazayeri, assistant to the Iranian chief-of-staff, told Iran's state-run Arabic-language Al-Alam TV that Tehran "will not allow the enemy [Israel] to harm the security of the region" and that "the resistance will retaliate against the Israeli aggression against Syria."

Israel has said it wants to stay out of the Syrian war on its doorstep, but Prime Minister Benjamin Netanyahu has repeatedly stated the Jewish state would be prepared to take military action to prevent sophisticated weapons from flowing from Syria to Hezbollah or other extremist groups.

Canada's Foreign Ministry

"We believe Israel has every right to defend itself and its people from terrorist threats. [Bashar] Assad continues his attempts to drag the entire region into his internal struggle to cling to power. Syria’s weapons are a concern to the entire international community."

—Rick Roth, press secretary for Foreign Affairs Minister John Baird

Israel and Hezbollah have a long history of enmity, and fought a punishing month-long war in mid-2006. The militant group fired thousands of rockets at Israel, while Israeli warplanes destroyed large areas of southern and eastern Lebanon during a conflict that ended in stalemate.

With Syria now engulfed in an internal conflict, Israel is especially concerned that Hezbollah will take advantage of the chaos and try to smuggle advanced weapons into Lebanon, particularly those that could hamper Israel's ability to operate in Lebanese skies.

Officials in Israel say they have identified several "game changing" weapons that would trigger military intervention in Syria: chemical weapons, long-range Scud B missiles, the Fateh-110s, land-to-sea Yakhont missiles and SA-17 anti-aircraft missiles.

Israeli officials believe that Iran and Hezbollah are stepping up their efforts to smuggle weapons because they think Assad's days are numbered. They also think the Syrian leader is co-operating because he is weak and he owes Hezbollah, which is understood to be sending men to fight alongside government troops, for its support.

The officials, who spoke on condition of anonymity because they were not authorized to brief the media, said Israel will continue to strike Syria if the smuggling attempts continue.

An amateur video said to be shot early Sunday in the Damascus area showed a massive explosion followed by fire lighting up the night sky. The video appeared genuine and corresponded to other AP reporting.

The state news agency said the Jamraya military and scientific research centre near Damascus was hit. It added that there were casualties but did not give a number.

Damascus-based activist Maath al-Shami said the strikes occurred around 3 a.m. local time and that one of them hit near Qasioun mountain that overlooks Damascus. He added that the raid targeted a military position for the elite Republican Guards unit that is in charge of protecting the capital.

Mohammed Saeed, another activist who lives in the Damascus suburb of Douma, said "the explosions were so strong that earth shook under us." He said the smell of the fire caused by the air raid near Qasioun was noticeable kilometres away.

Israeli security cabinet holds emergency meeting

In Jerusalem, Israeli media reported that Netanyahu held an emergency meeting of his inner security cabinet. The prime minister's office declined comment.

The strikes are a calculated risk for Israel. Officials there say they do not think Assad will retaliate in the immediate future, but they hedged their bets with the deployment of Iron Dome batteries to Israel's north.

Earlier this year, the Iron Dome system was credited with shooting down hundreds of rockets during a round of fighting against Hamas militants in the Gaza Strip.

Amos Yadlin, a former head of Israel's military intelligence, said the strikes on Syria are a signal to Damascus's ally, Tehran, that Israel is serious about the red lines it has set.

"Syria is a very important part in the front that Iran has built. Iran is testing Israel and the U.S. determination in the facing of red lines and what it sees is in clarifies to it that at least some of the players, when they define red lines and they are crossed, take it seriously," he told Army Radio.