Turkey has sealed its border with Syria to trucks, cutting off a vital supply line as fighting stretched into its fifth day in the city of Aleppo.

Stopping freight across the 566-mile border deprives Syria of a main route for imports and exports. Rebels fighting the regime of President Bashar al-Assad generally move their weapons and material over the border through clandestine smuggler routes.

Hayati Yazici, the Turkish Customs and Trade Minister, said the move came after rebels captured two border crossings between Syria and Turkey. Last week, dozens of Turkish trucks were either looted or burned when the rebels took the Bab al-Hawa crossing.

An alliance of rebel forces attacked Aleppo, Syria's commercial hub, on Saturday, infiltrating sympathetic neighbourhoods in the north and south and then gradually moving towards the historic old city at the centre, a UN world heritage site.

The Britain-based Syrian Observatory for Human Rights reported more than two dozen people killed in fighting in Aleppo and large numbers of people fleeing the southern neighbourhood of Sukkari.

Syrian forces, however, managed to quash a similar assault last week in the capital Damascus using heavy weapons including attack helicopters, which are now being deployed in Aleppo, according to local activists and residents. The government forces eventually overpowered the outgunned and outmanned rebels.

A new commander for the 300-member UN observer force, Lt. Gen. Babacar Gaye has arrived in Damascus along with the UN official in charge of peacekeeping operations to hold a series of meetings to assess the prospects for a UN peace plan that is being widely ignored.