International mediator Kofi Annan said on Monday he was horrified by the killings in the Syrian town of Houla.
He urged the Syrian government to take bold steps to show it was serious about reaching a peaceful solution to the country's crisis.
Speaking shortly after arriving in Damascus for talks with the government, Annan said he expected to have "serious and frank discussions" with Syrian President Bashar al-Assad.
An air strike by a Syrian plane in the northern rebel-held town of Azaz killed 30 people and wounded scores of others, a local doctor said.
The UN Special Envoy to Syria Kofi Annan has admitted he is "frustrated and impatient" over the continued violence and killing in Syria.
So after weeks of insisting his peace mission is the only game in town, it might be that Kofi Annan is preparing to pack up and leave.