Relief agencies have been evacuating 500 wounded people from four besieged Syrian towns, in what has been described as the largest such operation so far in the five-year conflict, BBC News reports.

Half are coming from towns blockaded by pro-government forces, and half from towns blocked off by rebels.

A UN chief in Syria said he had been told the operation had been successfully carried out.

Meanwhile, peace talks that started in Geneva a week ago are in difficulty. It is not clear whether a partial truce that has lasted seven weeks so far will hold.

Opposition representatives walked out of the talks this week, blaming government violations of the ceasefire.

Activists said government air strikes on Tuesday had killed 40 people in rebel-held areas.