Syria’s allies on Thursday condemned an Israeli airstrike a day earlier on a Syrian target, and Iran issued a veiled threat to respond, but Israeli analysts said the likelihood of retaliatory attacks across Israel’s borders was slim.

As Israeli officials voiced rising concern this week about the security of Syria’s chemical weapons, the military moved missile defense batteries to northern Israel and anxious Israelis lined up at some gas-mask distribution centers. But Israeli authorities took no extraordinary measures to prepare for a possible response after Wednesday’s strike, apparently calculating that the turmoil in Syria and Israel’s military deterrence would discourage any hostilities.

Details of the attack remained murky, and the United States and Israel maintained a public silence in what analysts said was probably a calculated decision to reduce the prospect of retaliation. Western officials and a former Lebanese security official initially said that the strike was carried out near Syria’s border with Lebanon and possibly hit a truck ferrying weapons, but Syria later announced that a defense research center near Damascus, the capital, had been bombed.

The statement from Syria prompted allies to spring to its defense Thursday. Iran’s deputy foreign minister, Hossein Amir-Abdollahian, was quoted by Iranian news agencies as warning Israel of “grave consequences” after the airstrike. Analysts described the attack as a probable attempt by Israel to stop the transfer of sophisticated antiaircraft weapons from strife-torn Syria to the Iranian-backed Hezbollah militant group in Lebanon.

As the civil war in Syria continues, refugees are streaming into Turkey. The Post’s Jenna Johnson just returned from the region and describes what she saw. (The Fold/The Washington Post)

Hinting at possible retaliation, Amir-Abdollahian said Israel should not rely on its Iron Dome missile shield, which he said proved ineffective during Israel’s military offensive in November against the militant Hamas group in the Gaza Strip. The system stopped scores of incoming rockets, but others struck southern Israeli cities and reached areas near Jerusalem and Tel Aviv.

Russia, Syria’s most powerful ally, said in a statement that if the strike is confirmed, “we have a case of unprovoked attacks on targets in the territory of a sovereign state, which grossly violates the U.N. charter and is unacceptable.’’

In Washington, outgoing Secretary of State Hillary Rodham Clinton declined Thursday to discuss the Israeli strike but said Iran and Russia were continuing — and, in Iran’s case, increasing — their assistance to Syrian President Bashar al-Assad.

Clinton dismissed Russian Prime Minister Dmitry Medvedev’s call this week for negotiations between Assad and his opponents. The Russians, she said in an interview with State Department correspondents, are not “passive bystanders in their support for Assad” and are continuing to send financial aid and military equipment to his government.

Iran, Clinton said, had expanded the number of fighters, trainers and supplies it is sending to the Syrian military. “We know that the Iranians are all in for Assad,” she said.

Syria’s ambassador to Lebanon, Ali Abdul Karim Ali, was quoted on Hezbollah’s al-Ahd news Web site as saying that Syria could opt for a “surprise” retaliation, but he added that he could not predict when such a response might come. Hezbollah called the Israeli strike “savage aggression” and expressed “full solidarity with Syria’s leadership, army and people,” but it did not threaten to retaliate.

Hezbollah, which defense analysts say has expanded its arsenal to include as many as 50,000 missiles, has avoided firing rockets across the border since a war with Israel in 2006 that devastated the group’s strongholds in southern Lebanese villages and parts of Beirut.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.

Despite the tough rhetoric from Syria and its allies, Israeli analysts said Syria’s precarious position, which has also weakened its ally Hezbollah, made retaliation unlikely.

“Syria is in total military disarray,’’ said Itamar Rabinovich, a former Israeli ambassador to Washington who was Israel’s chief negotiator with Syria in the 1990s. “The Syrian army can’t cope with ragtag bands of rebels, so it’s not up to taking on Israel. Hezbollah is in a delicate position in Lebanon, where it has been weakened by its unpopular support for the Syrian regime. Getting Lebanon in trouble again is not what they need.”

Shlomo Brom, a senior research fellow at the Institute for National Security Studies in Tel Aviv, said Syria was not likely to risk further damage to its military through a confrontation with Israel. Hezbollah, he added, has similar concerns.

“If Hezbollah launches rockets at Israel, it will start a large conflict whose cost will be borne mostly by Lebanon and its citizens,” Brom said. “The last thing they need is to be accused of causing the destruction of Lebanon.”

Iran also has an interest in avoiding a military clash with Israel that could lead to strikes on its nuclear facilities, Rabinovich said. “They have other ways of striking back, encouraging terrorist activity here or around the globe.”

A Syrian army statement after Wednesday’s strike stopped short of threatening retaliation for what it called a “flagrant violation of Syrian sovereignty and airspace.” Instead, Syria’s Foreign Ministry on Thursday lodged a protest with the head of the United Nations’ observer force stationed between Syrian and Israeli troops on the Golan Heights. It also sent a letter to the U.N. Security Council, urging it to condemn the attack.

A report by the official Syrian Arab News Agency quoted Prime Minister Wael al-Halki as saying that rebel attacks on air defense and radar posts had laid the groundwork for the Israeli strike.

“This criminal attack goes in line with the conspiracy targeting Syria, and the timing of this attack provides evidence of the coordination and the unity of stands between what is done by the Israeli enemy and what is done by the terrorist armed groups,” the report quoted Halki as saying. The Syrian government refers to rebel fighters as terrorists.

U.N. Secretary General Ban Ki-moon expressed “grave concern” about the reports of the Israeli attack, which he said the body had not confirmed, and warned against “escalation in the region.”

Babak Dehganpisheh in Beirut and Karen DeYoung in Washington contributed to this report.