Some 280,000 people are estimated to have been killed in Syria since the conflict began in 2011.

A partial ceasefire took effect in February as part of a negotiated deal based on a UN resolution, in order to end the conflict and establish a transitional government.

A new round of peace talks officially began in January, but substantive negotiations did not take place until March.

Since then, the ceasefire has been repeatedly broken with fighting continuing in many parts of the country. Opposition delegates walked out of the talks in April. The UN says conditions for a third round of talks still do not exist.