The UN's human rights office warned on Friday of an "imminent" showdown between government troops and opposition forces in Syria's second-largest city, Aleppo.

The rebels have been locked in fierce fighting with government troops in Aleppo for six days and are bracing for an attack amid reports that the regime is amassing reinforcements to retake the city of 3 million.

Expressing her "deep alarm" at the situation, UN High Commissioner for Human Rights Navi Pillay said the reports coming out of Damascus, "along with the reported build-up of forces in and around Aleppo, bodes ill for the people of that city".