Britain Calls for Immediate Ceasefire in Syria's Aleppo

Britain called on Monday for an immediate ceasefire in Aleppo, saying a humanitarian catastrophe threatened the Syrian city.

Foreign Secretary Boris Johnson also urged Russia and Iran to use their influence on Syrian President Bashar Assad to stop the assault his forces are waging.

Syria's rebels lost all of the northern neighborhoods of their stronghold in east Aleppo on Monday, as the regime army made significant advances in its offensive to recapture the entire city.

"I call on those with influence on the regime -- especially Russia and Iran -- to use this to end the devastating assault on eastern Aleppo and ensure the U.N.'s humanitarian plan for the city can be implemented," Johnson said in a statement.

"The assault is threatening a humanitarian catastrophe. Hundreds of civilians are reported killed in the past fortnight and thousands displaced by attacks over the weekend.

"We need an immediate ceasefire in Aleppo and immediate access for impartial humanitarian actors to ensure the protection of vulnerable civilians fleeing the fighting. These are humanitarian imperatives."

Johnson said everyone involved in the assault had a responsibility to protect civilians under international humanitarian law.

"Elsewhere, the Assad regime is using siege and bombardment tactics to force surrenders -- this too must stop," he added.

The loss of eastern Aleppo would be a potentially devastating blow to Syria's rebels, who seized the area in 2012.