U.N. envoy Kofi Annan arrived in Damascus Monday for talks with President Bashar Assad and other government officials.
The opposition said government forces launched an attack on Friday that killed more than 100 people in Houla. Many of them were young children.
They said the attack began with artillery and tank shelling, and then militia thugs shot men at close range and stabbed women and children. The U.N. counted 49 children and 34 women among the dead.
Annan said he is horrified by the violence. Even Syria's ally, Russia, has joined international criticism over the mass killing.
The protests against Assad began in March 2011 and turned into an uprising after his government responded with a violent crackdown on dissent. The U.N. estimated that at least 9,000 people were killed in the first year of the conflict, but hundreds more have been killed since then.
The Associated Press contributed to this report.