Syria's threat that it will use chemical and biological weapons against a foreign attack has triggered concern among Israeli officials who have discussed the possibility of a Syrian government collapse with US envoys.

If the Assad regime crumbles, Syria's stockpile of chemical weapons could fall into the hands of Islamist militants, Israel has warned the US.

"You can presume that these sorts of issues came up with American officials when they visited recently in Israel," an unnamed official told AP.

Israeli prime minister Benjamin Netanyahu warned that his country would "have to act" to safeguard the arsenal and refused to rule out military intervention.

"Could you imagine Hezbollah, the people who are conducting with Iran all these terror attacks around the world - could you imagine them having chemical weapons? It would be like al-Qaida having chemical weapons," he told Fox News.

Syria has reassured international leaders that it will not use its chemical or biological weapons against its own citizens.

"No chemical or biological weapons will ever be used during the crisis no matter what the developments [are] inside Syria," the foreign ministry's Jihad Makdissi said on Syrian state TV.

"All of these types of weapons are in storage [securely] and under the direct supervision of the Syrian armed forces and will never be used unless Syria is exposed to external aggression."

Syria's arsenal includes nerve agents, mustard gas, missile systems capable of delivering them and a variety of advanced conventional arms, including anti-tank rockets and late-model portable anti-aircraft missiles.

In other developments, Iraqi prime minister Nouri al-Maliki formally authorised the admission of Syrian refugees into the country.

Fierce clashes between Assad's forces and rebels are continuing in Aleppo, Syria's largest city.

The Arab League has agreed to offer Assad safe passage if he resigns. But Iraq has refused to back the offer, saying that foreign governments should "not interfere".