Shots ring out through the streets of Syria's largest city, Aleppo.

A couple of seconds later - what appears to be heavy artillery slams into a building nearby - sending up a cloud of smoke.

This video, which comes from a social media website, purports to show rebels in pitched battles with Syrian government troops.

Activists say the army has launched a major offensive to retake parts of the city seized by insurgents earlier this month.

This video appears to show a helicopter circling overhead - part of what observers have called a massing of machinery and heavy weapons by forces loyal to President Bashar al-Assad.

Reuters is not able to independently verify the content of such videos.

The battle for Aleppo, a city of 2.5 million people, is seen as a crucial test for a government that has struggled to contain an uprising now a year-and-a-half old.

Andrew Raven, Reuters