Once a Syrian ally, Turkey has become one of the regime's harshest critics, and now shelters many of the opposition activists and defectors, including army officers, who have switched to the rebel side.
Syria's conflict started 22 months ago as an uprising against Assad, whose family has ruled the country for four decades. It quickly turned into a civil war, with rebels taking up arms to fight back against a brutal government crackdown.
Despite recent loses of army bases and large swathes of land in the north along the border with Turkey, the regime has managed to keep its grip on the country in large part due to Assad's airpower.
Also Wednesday, Human Rights Watch reported that armed opposition groups appeared to have deliberately destroyed religious sites in mixed areas of northern Syria in the last two months of last year.
The New York-based group said investigations showed an armed opposition group destroyed two churches in the coastal region of Latakia and a Shiite Muslim place of worship in the northwestern province of Idlib. Evidence and witness testimony suggested that all three attacks took place after the areas fell to opposition control and government forces had left, the group said.
Assad's regime is dominated by members of his minority Alawite sect, an off-shoot of Shiite Islam, while most of the rebels are Sunni Muslims.
Mainly Sunni Islamic extremists have joined the rebels in their fight against Assad, including Jabhat al-Nusra. The U.S. says the group is linked to al-Qaida, and has declared it a terrorist organization.
Human Rights Watch previously documented the destruction and looting of a mosque in the town of Taftanaz in Idlib province by Syrian government forces.
"The destruction of religious sites is furthering sectarian fears and compounding the tragedies of the country, with tens of thousands killed," said Sarah Leah Whitson, Middle East director at Human Rights Watch. "Syria will lose its rich cultural and religious diversity if armed groups do not respect places of worship. Leaders on both sides should send a message that those who attack these sites will be held accountable."
Isachenkov reported from Moscow.