Israeli attacks are a 'declaration of war' says Syrian deputy foreign minister Faisal al-Mekdad after second strike in three days

Israel has carried out a second series of air strikes in three days on Syria, this time hitting targets close to the capital, Damascus, and raising further fears of the conflict spreading across the region.

There were early reports that a military and scientific research centre at Jamraya, which the opposition claims has been used to produce chemical weapons, was among the locations attacked. State media reported that there had been a number of casualties.

However, it appears that the targets may have been stockpiles of Iranian-made Fateh-110 missiles which, Tel Aviv believes, may have been heading across the border to the Hezbollah militia. The first bombings, 48 hours earlier, were aimed at batches of the same weapons whose presence in Lebanon was said to have been described by the Israeli premier Benjamin Netanyahu as a "game-changer".

Syria's Deputy Foreign Minister said the attacks were a "declaration of war" and that they invited retaliation. Faisal al-Mekdad warned "Israeli aggression opens the door to all possibilities", and that action would be taken "in our own time and way". The regime, which has claimed that an international conspiracy is trying to dismember the country by supporting the rebels, said in a statement that the air strikes were "direct support for terrorists".

United Nations Secretary-General Ban Ki-moon on Sunday expressed "grave concern" at the reports of Israeli strikes, but said the UN was unavailable to independently verify the incidents.

Ban's press office said he "calls on all sides to... act with a sense of responsibility to prevent an escalation of what is already a devastating and highly dangerous conflict". The Syrian National Coalition opposition group condemned Israeli strikes inside Syria, and said the Jewish state had "taken advantage" of the conflict.

Amateur video footage in Damascus showed flames lighting up the night sky and sounds of multiple explosions lasting several minutes in the vicinity of Mount Qasioun, which houses a number of military installations.

Fighters from Hezbollah have been battling alongside the forces of the Syrian President Bashar al-Assad in the country's civil war. Israel, which fought an inconclusive war with the group seven years ago, has accused it of building up an arsenal of long-range missiles which can reach deep into the Jewish state.

The Fateh-110, with an elaborate guidance system, and sufficient range to reach Tel Aviv from south Lebanon, is much more sophisticated that anything held by Hezbollah so far, and large shipments are said to have arrived from Iran in the past three months.

Israeli military analysts say there are fears that Scud Ds – which, with a range of 422 miles, would be able to hit southern Israel from Lebanon – may also be destined for Hezbollah. The Israelis also carried out bombings inside Syria in January against a convoy carrying SA-17 anti-aircraft systems en route to Lebanon. The Israeli government has stated that it will not allow the delivery of advanced missiles to Hezbollah, using similar rhetoric to Barack Obama who declared that the Syrian regime's use of chemical weapons would be a "red line".

No definite proof has so far emerged that weapons of mass destruction have actually been used in the conflict and the indications from Washington are that the US administration is keen to avoid direct military action. But military analysts point out that the raids by Israeli warplanes, and the Assad regime's inability to counter them, show that one argument against imposing a no-fly zone – the supposed effectiveness of Syrian air defences – could be brought into question.

Speaking shortly before yesterday's raids, President Obama insisted that Israel had a right to act. "The Israelis have to guard against the transfer of advanced weaponry to a terrorist organisation like Hezbollah," he said.

Iran urged neighbouring countries to unite "in the face of Zionist aggression" and stated it was prepared to train President Assad's forces to resist foreign attacks.

The targets may have been a cache of advanced missiles destined for Hezbollah