Reports that the Syrian government may be considering the use of chemical weapons have also heightened international concerns in recent days. Syria denies it has such weapons and says it would not use them even if it did.
The Syrian civil war started in March 2011 when a government crackdown on civilian demonstrators morphed into a fight between the regime and rebels.
The conflict has a proxy element, with Sunni countries such as Turkey, Qatar and Saudi Arabia backing the rebels and Shiite Iran backing the Alawite regime. The Alawite faith is an offshoot of Shiite Islam.
More than 40,000 people have died in the war. The United Nations said on Friday that many Syrians will continue to be killed and maimed after the war ends because of deadly explosives placed in residential areas across the country.