The leader of Syria’s most powerful insurgent group has toured the seized city of Aleppo in a surprise visit, for the first time since the group captured large parts of the city last weekend, as fierce fighting intensifies in the government-led counter-offensive in northern Hama.

Islamist rebels have been advancing through large swathes of the country's north-west in the past week, threatening the rule of Syria's long-time president, Bashar Al-Assad.

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.