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.

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