Iranian-backed militias entered Syria overnight from Iraq and were heading to northern Syria to beef up beleaguered Syrian army forces battling insurgents, according to two Syrian army sources.

Dozens of Iran-aligned Iraqi Hashd al Shaabi fighters from Iraq also crossed into Syria through a military route near Al Bukamal crossing, a senior Syrian army source told Reuters.

"These are fresh reinforcements being sent to aid our comrades on the front lines in the north," the officer said, adding the militias included Iraq's Katiab Hezbollah and Fatemiyoun groups.

Iran sent thousands of Shi'ite militias to Syria during the Syrian war and, alongside Russia with its air power, enabled Syrian President Bashar Assad to crush the insurgency and regain most of his territory.

A lack of manpower to help thwart the rebel onslaught in recent days contributed to the speedy retreat of Syrian army forces and withdrawal from Aleppo city, according to two other army sources. Militias allied to Iran, led by Hezbollah, have a strong presence in the Aleppo area.

Israel has also in recent months stepped up its strikes on Iranian bases in Syria while also waging an offensive in Lebanon which it says has weakened Hezbollah and its military capabilities.