An accidental explosion in a secret weapons facility in Syria killed dozens of Syrian and Iranian military engineers as they were attempting to mount a chemical warhead on a Scud missile, according to an authoritative military journal.

Fifteen military personnel and “dozens” of Iranian advisers died when the fuel for the missile caught fire and the weapon exploded, according to unnamed Syrian sources quoted by Jane’s Defence Weekly.

The report said that the explosion sent out a cloud of chemical and nerve gases, including the deadly VX and Sarin agents as well as mustard gas, across the facility in the northern city of Aleppo. The claims could not be verified independently by sources in London and the United States.

The official Syrian news agency, Sana, reported that 15 Syrian military personnel were killed and 50 others injured in an accident involving “very explosive products” on July 26. It made no mention of Iranian officers also being killed in the blast, which it said was not an act of sabotage.

Jane’s claimed that the engineers were trying to weaponise a Syrian-made Scud missile with a range of about 300 miles (480km) when the explosion occurred. The Syrians and Iranians are thought to have been working closely together on developing a more effective chemical warhead for the Scud ballistic missile system.

Both Syria and Iran have benefited from an advanced version of the Scud, which was designed by the North Koreans. Duncan Lennox, editor of Jane’s Strategic Weapon Systems, said that North Korea had managed to reduce the weight of the missile and increase the strength of the outer casing, as well as change some of the components inside. The technology was passed on to Syria and Iran, and “they put their heads together”, Mr Lennox said.

Reports of the accident coincided with widespread speculation that an audacious Israeli airstrike against another Syrian facility two weeks ago may have destroyed a nuclear shipment from North Korea.

Although Israel has maintained an uncharacteristic silence, US officials led by John Bolton, the hawkish former Ambassador to the United Nations, have hinted that North Korea might have been trying to hide nuclear materials in Syria to avoid inspection. No proof has so far been offered to support the claims, which Syria and North Korea deny and which critics say recall the whispering campaign about weapons of mass destruction that preceded the US invasion of Iraq in 2003.

If the claims are true, the Israeli strike would be the most daring long-range mission launched since it destroyed the Iraqi nuclear plant being built by Saddam Hussein at Osiraq, near Baghdad, in June 1981.

Moshe Maoz, an Israeli expert on Syria, said it was possible the strike involved some kind of nuclear material. He said it would most likely have been an imported warhead, because Damascus lacks the infrastructure to develop its own nuclear programme. “Israel wouldn’t go for such a bold, daring and dangerous action without a very good reason,” he said. “My guess is that it is something to do with nuclear materials.”

General Mohammad Alavi, the deputy commander of the Iranian Air Force, said yesterday that Tehran would strike back if Israel launched any raids on its territory. “We have drawn up a plan to strike back at Israel with our bombers if this regime makes a silly mistake,” he said.

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