The Iron Dome deployment followed a surprise Israeli drill last week in which several thousand reservists simulated conflict in the north. In another possible sign of concern, Israel closed the airspace over northern Israel to civilian flights on Sunday and tightened security at embassies overseas, Israeli media reported. Israeli officials would not confirm either measure.
Reflecting fears of ordinary Israelis, the country's postal service, which helps distribute government-issue gas masks, said demand jumped to four times the normal level Sunday.
Israel's deputy defense minister, Danny Danon, would neither confirm nor deny the airstrikes. He said, however, that Israel "is guarding its interests and will continue to do so in the future."
"Israel cannot allow weapons, dangerous weapons, to get into the hands of terror organizations," he told Army Radio.
Israeli defense officials have identified several strategic weapons that they say cannot be allowed to reach Hezbollah. They include Syrian chemical weapons, the Iranian Fateh-110s, long-range Scud missiles, Yakhont missiles capable of attacking naval ships from the coast, and Russian SA-17 anti-aircraft missiles. Israel's airstrike in January destroyed a shipment of SA-17s meant for Hezbollah, according to U.S. officials.
Israeli officials said Sunday they believe that Iran is stepping up its efforts to smuggle weapons through Syria to Hezbollah because of concerns that Assad's days are numbered.
They said the Fateh-110s reached Syria last week. Friday's airstrike struck a site at the Damascus airport where the missiles were being stored, while the second series of airstrikes early Sunday targeted the remnants of the shipment, which had been moved to three nearby locations, the officials said.
None of the Iranian missiles are believed to have reached Lebanon, said the officials, who spoke on condition of anonymity because they were discussing a classified intelligence assessment.
The attacks pose a dilemma for the embattled Assad regime.
If it fails to respond, it looks weak and opens the door to more airstrikes. But any military retaliation against Israel would risk dragging the Jewish state and its powerful army into a broader conflict. With few exceptions, Israel and Syria have not engaged in direct fighting in roughly 40 years.
The airstrikes come as Washington considers how to respond to indications the Syrian regime may have used chemical weapons in its civil war. President Barack Obama has described the use of such weapons as a "red line," and the administration is weighing its options.
The White House declined for a second day to comment directly on Israel's air strikes in Syria, but said Obama believes Israel, as a sovereign nation, has the right to defend itself against threats from Hezbollah.
Iran condemned the airstrikes, and a senior official hinted at possible retribution from 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." He added that "the resistance will retaliate to the Israeli aggression against Syria." ''Resistance" is a term used for Hezbollah and the Palestinian Hamas, another anti-Israel militant group supported by Iran.
Iran has provided both financial and military support to Hezbollah for decades and has used Syria as a conduit for both. If Assad were to fall, that pipeline could be cut, dealing a serious blow to Hezbollah's ability to confront Israel.
U.N. Secretary-General Ban Ki-moon spoke to Arab League Secretary-General Nabil ElAraby by telephone Sunday and both shared their "grave concern" over the air strikes, U.N. spokesman Martin Nesirky said.
Ban called on all sides "to exercise maximum calm and restraint, and to act with a sense of responsibility to prevent an escalation of what is already a devastating and highly dangerous conflict," Nesirky said.