Syrian army forces quit Hama as rebels entered the northern city, dealing another heavy blow to the embattled government which has been propped with aid from Iran and its regional allies for over a decade.

Syrian forces were withdrawn "to preserve the lives of civilians and to prevent urban combat", the army said in a statement, as Al Jazeera broadcast footage of opposition fighters in the city center.

The shock advance of hardline Islamist-led rebels wrested the second largest city Aleppo from the forces of President Bashar al-Assad last week, endangering the Islamic Republic's oldest Arab ally in region.

A rebellion which has festered for over a decade since Arab Spring protests in 2011 had mostly been quelled after heavy backing from Iranian paramilitary forces and fighters from the Lebanon's Tehran-backed Hezbollah militia.

Iran has said it is ready to support Assad and has returned to the country a top Islamic Revolutionary Guard Corps commander who had helped Damascus wrest Aleppo back from the rebels in 2016.

"We firmly stand by Syria and will certainly intervene in the fight against terrorists”, Abbas Golroo, an Iranian lawmaker who recently visited Damascus said on Thursday.

"The Islamic Republic of Iran stood by Syria for ten years and now too has responded to the Syrian government’s request to intervene," he added. "Our intervention is aimed at preventing the presence of terrorists and combating them.”

Iraqi militias long armed and funded by Iran have been entering the country since the rebel offensive began, Reuters reported quoting Syrian military sources.