Armed opposition groups in Syria appear to have looted Christian churches and destroyed a Shiite Muslim mosquee in recent months, New York-based Human Rights Watch said Wednesday.
The rights group said it had previously documented the destruction of a Sunni mosque in Taftanaz by government troops fighting for President Bashar Assad.
The war has already killed more than 60,000 Syrians, according to U.N. estimates.
Human Rights Watch warned an increase in sectarian violence can only make things worse.
"The destruction of religious sites is furthering sectarian fears and compounding the tragedies of the country," Sarah Leah Whitson, Middle East director for Human Rights Watch, said in a statement.
"Leaders on both sides should send a message that those who attack these sites will be held accountable," she added.
Sunni Muslims make up about three-quarters of the population, and most of the rebel fighters are Sunnis, according to the CIA's World Factbook.
Assad, however, is a member of the Alawite sect, which is more closely linked to Shiite Islam. Many of his appointees in high government and the military are also Alawites.
Human Rights Watch noted that international humanitarian law requires warring parties to avoid deliberate targeting or seizure of religious buildings that aren't being used for military purposes.
The group said it found evidence in three villages of attacks against religious sites after opposition groups had taken over and driven out government forces. In each area, religious minorities had fled in large numbers, if not entirely.
In Zarzour, majority Sunnis told the group that their Shiite neighbors fled because they feared they would be attacked by opposition fighters if there was a perception that they had been supportive of government forces.
The Sunni villagers told Human Rights Watch that the Shiites had given "preferential treatment" to government forces when they were in Zarzour.
The rights group said its observations and witness accounts indicated that opposition fighters deliberately started a fire in a Shiite mosque when it took over the village.
In Jdeideh, local residents told observers that gunmen "operating in the name of the opposition" had broken into and stolen from a Christian church after the area came under rebel control.
Observers from the group said it appeared that gunmen had broken in, stolen from the church and fired numerous shots inside, shattering windows and causing structural damage.
A villager told observers that the fighters had used the adjacent priest's quarters to fire at government forces and had stolen medicine from a clinic owned by the church, looted homes and kidnapped civilians.
The rights group said it could not determine whether there was a religious motive for any looting or kidnapping.
In a third village, Ghasaniyeh, the group found that a local church had been broken into and gasoline and diesel fuel had been stolen. An observer found that the church doors had been forced open and that a cross had been left on the floor, but the group said the building otherwise was undamaged.
"The opposition of Syria should back up its claims that it will uphold minority rights by protecting places of worship," Whitson said in her statement.
Calls and emails to the National Coalition of Syrian Revolution and Opposition Forces and a representative of the Syrian National Council were not immediately returned.