UN Syria envoy Staffan de Mistura has said he fears the fragile truce in Syria is collapsing, BBC News reports.
In the latest violence, 18 people were reported to have died in government air strikes in Aleppo and a Syrian warplane was said to have crashed near Damascus.
Speaking in Geneva, Mr de Mistura said he wanted to call a regional summit to prevent peace talks failing.
He said he thought that 400,000 people may now have died in the conflict.
The UN diplomat said he had "no proof" for the figure but said he feared it was not far off.
