The Syrian government said that Israeli air strikes against military targets around Damascus amounted to a "declaration of war" and threatened retaliation, in the latest sign that the fighting is spilling across the Syrian border and risks sparking a wider regional conflict.

Israel made no official comment on the strikes early on Sunday, which were the second in two days and the third and heaviest this year. Security sources said they were aimed at preventing the transfer of advanced Iranian-made missiles to Hezbollah in Lebanon for possible use against Israel.

After the attack, Israel deployed two batteries of Iron Dome anti-ballistic missiles, designed to intercept incoming enemy missiles, to the north of the country, and the prime minister, Binyamin Netanyahu, delayed a trip to China to chair a meeting of his security cabinet.

The Iranian army's ground forces commander, Brigadier General Ahmad-Reza Pourdastan, said Iran was ready to train the Syrian army if necessary, something Israeli and western officials say has been going on for some time, but observers said that the increasingly public and bellicose declarations from Syria's neighbours showed the conflict's potential for spreading.

In a further development, UN human rights investigators have said they have "strong suspicions" that Syrian rebel forces might have used the nerve agent sarin.

Carla del Ponte, one of the lead investigators, said the UN independent commission of inquiry on Syria has not yet seen evidence of government forces having used chemical weapons, which are banned under international law. But she told Swiss-Italian television: "Our investigators have been in neighbouring countries interviewing victims, doctors and field hospitals and, according to their report of last week, which I have seen, there are strong, concrete suspicions but not yet incontrovertible proof of the use of sarin gas, from the way the victims were treated. This was use on the part of the opposition, the rebels, not by the government authorities."

Del Ponte gave no details as to when or where sarin may have been used. The report followed claims last month that president Bashar al-Assad had used sarin gas in the conflict, but there has so far been no proof of its use.

The Damascus regime's deputy foreign minister, Faisal al-Miqdad, told CNN the Israeli air strikes at the weekend represented "a declaration of war" and betokened an alliance between Islamist terrorists and Israel. He said Syria would retaliate in its own time and in its own way.

Omran Zoabi, the information minister, said: "Syria is a country that does not accept insults and it doesn't accept humiliation."

Israeli military analysts said the missiles had been fired from outside Syrian airspace to avoid engaging Syria's reportedly formidable air defences. The Lebanese army said that Israeli planes had flown above Lebanon, an act that drew condemnation from the country's president, Michel Suleiman.

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The office of the Egyptian president, Mohamed Morsi, also denounced the attack, declaring it illegal and a threat to "security and stability in the region". Meanwhile, Nabil Elaraby, chief of the Arab League, appealed to the UN security council to "move immediately to stop the Israeli aggressions on Syria".

The air strikes lit up the night sky above west Damascus in the early hours of Sunday. Witnesses described a string of blasts that caused fiery clouds in the sky above Mount Qassioun, from where government artillery has been pounding rebel-held areas. "The explosion was very, very strong," a Damascus-based activist, Maath al-Shami, told the Associated Press.

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 detectable kilometres away.

Syrian state media reported that Israeli missiles had hit a military and scientific research centre in Jamraya near Damascus and caused casualties. Syrian officials had claimed that the first Israeli missile strikes in January had hit the same target, but that was denied at the time by US officials, who said the raids had been aimed at a missile shipment intended for Hezbollah.

While avoiding direct confirmation that Israel had struck, Shaul Mofaz, a former defence minister, told Israel Radio: "The policy of preventing leakage of significant weaponry and advanced systems to Hezbollah is right, otherwise we could encounter it here in Israel."

A senior Israeli official was quoted by AP as saying the air strikes were aimed at destroying Fateh-110 missiles, a solid-fuelled Iranian weapon with a 200 mile range and precision guidance systems, far more effective than anything in Hezbollah's existing arsenal. Its Farsi name means "conqueror".

Michael Herzog, a retired Israeli brigadier general, said: "The context according to reports I have seen is similar to January: a shipment of strategic weapons which would be a game-changer in a conflict with Hezbollah. There is great concern here about the spillover from Syria and particularly about strategic weapons – not just chemical weapons, but also missiles.

"Israel did act and will act whenever it feels its national security interests are threatened, be it on the joint border with Syria – which has been quiet until now but may not remain that way – or with the transfer of strategic weapons to Hezbollah."

When Barack Obama visited Israel in March, Netanyahu asked for US help in stopping the spread of Syrian missiles and chemical weapons. "These missiles are not just a problem for Israel," a senior Israeli official told the Guardian.

Israeli officials also acknowledged that such air strikes could spark a new, highly destructive cross-border war with Hezbollah.

Emile Hokayem, a regional analyst at the International Institute for Strategic Studies, said there were credible reports that, following the Israel-Hezbollah war of 2006, advanced Iranian missiles intended to help rearm the Lebanese Shia militia had been stockpiled in Syria under a joint custody arrangement.

"In 2006-07 Hezbollah was restocking obviously and the Syrians had an interest in a conflict-management role to keep some control," Hokayem said. "This quality weaponry was prepositioned in Syria, under some kind of joint custody. We don't know the exact mechanisms, but it would clearly have been a very dangerous sovereignty for Assad to allow the Hezbollahis and Iran to do this without Syrian control."

Amos Yadlin, a former chief of military intelligence who heads the Institute for National Security Studies in Tel Aviv, told Army Radio that Syria risked serious damage to its already battered military capabilities if it responded to the latest air strikes.

"Assad knows that the rebels have made him the primary target, and if he tries to deflect the fire towards Israel, chances are that he will be attacked by both the rebels and Israel," Yadlin said.

The air strikes were also a signal to Iran, Yadlin said, making it clear to Tehran that "when at least some of the players define red lines, and they are crossed, they take it seriously".

Netanyahu has urged the US and other nations to set a "red line" for Iran's nuclear programme, beyond which it could face military strikes on facilities Israel says are developing the components of a nuclear weapon.