Israel's military intelligence chief on Thursday said the regime of Syrian President Bashar al-Assad has contingency plans to use chemical weapons as it battles insurgents.

Speaking at Israel's annual Herzliya Conference on security and policy, Major General Avi Kohavi said Assad was still in control of his stockpile of chemical weapons, which the Jewish state fears could fall into the hands of militants.

"Assad is making advance preparations to use chemical weapons. He did not give the order yet, but is preparing for it," he told delegates, while stressing that the Syrian leader was still in control of Damascus's chemical stockpiles, military hardware and the air force.

Addressing the same conference earlier this week, army chief Benny Gantz warned that "terrorist" groups fighting alongside insurgents seeking to bring down the Assad regime were becoming stronger.

"The situation in Syria has become exceptionally dangerous. The terrorist organisations are becoming stronger on the ground. Now they are fighting against Assad but in the future they could turn against us," Gantz said.

Several radical Islamist groups have joined the two-year uprising against Assad, with Al-Nusra Front being the most prominent.

Kohavi also said Assad's collapse would harm his allies, Iran and Lebanon's powerful Hezbollah militant group.

When Assad falls, Iran "will lose the ability to transfer weaponry through Syria to Hezbollah," he said.

"Iran and Hezbollah are both doing all in their power to assist Assad's regime. They support Assad operationally on the ground, with strategic consultation, intelligence, weapons," the intelligence chief told delegates.

"They are establishing a popular army trained by Hezbollah and financed by Iran, currently consisting of 50,000 men, with plans to increase to 100,000,' he said.

"Iran and Hezbollah are also preparing for the day after Assad's fall, when they will use this army to protect their assets and interest in Syria."