Syrian and Russian jets are stepping up strikes on opposition forces in northern Syria in retaliation for the sudden offensive that has cost the regime control of the country’s second largest city, Aleppo.

Syrian rebel fighters began preparations to seize Aleppo a year ago but the assault was delayed by war in Gaza and ultimately launched last week when a ceasefire took hold in Lebanon, the head of Syria's main opposition abroad told Reuters.

Rebels captured Syria's second-largest city, Aleppo, over the weekend and advanced toward Hama in a surprise incursion that reignited Syria's 13-year-old civil war and embarrassed President Bashar al-Assad's forces.

The Syrian government has been fighting back against opposition forces with airstrikes after they took over large swathes of land in a lighting advance which has caused embarrassment to President Bashar Al-Assad.

Syria carried out airstrikes targeting the rebel forces that seized the country's second largest city of Aleppo in a surprise advance last week that caught the government off guard.