Leaders plea for an end to Syrian assault on Aleppo

World leaders have called on Syria's government to end its assault on the country's biggest city, Aleppo.

Thousands of troops have surrounded the area, parts of which have been seized by rebel fighters.

The United Nations, along with Britain, the United States and France, have said they fear a full-scale attack on the city, and the mass slaughter of civilians.

The BBC's Jeremy Bowen reports.