United Nations Secretary-General Ban Ki-moon expressed "great shock" on Tuesday over reports of a massacre in the Syrian village of Maan.
A statement released by Ban's spokesman Martin Nesirky said the UN leader was disturbed by reports of killings in Maan on February 9.
"Dozens of civilians are said to have been brutally killed," the statement said.
"The Secretary-General condemns in the strongest terms all violence against civilians and calls for the perpetrators of this massacre, and all other crimes in Syria, to be brought to justice."
The London-based Syrian Observatory for Human Rights said at least 25 members of Syrian leader Bashar Al-Assad's Alawite sect were killed by Islamist fighters in Maan, in Hama province, on Sunday.
The group said most of the dead were pro-regime militiamen, but state television reported a "massacre" of 10 women.
Ban's statement said the reported massacre "should be a reminder to all of the urgency of ending the conflict and launching a political transition towards a new Syria where all people and communities are guaranteed protection, rights and freedoms."