Kofi Annan told President Bashar al-Assad of the world's "grave concern" about violence in Syria, including the Houla massacre, in a meeting on Tuesday, the peace envoy's office said.
Annan met Assad for crunch talks in Damascus as several European nations and Canada expelled Syrian diplomats in protest at last week's massacre of at least 108 people in a village near the central town of Houla.
In the meeting, the UN-Arab League envoy conveyed "the grave concern of the international community about the violence in Syria, including in particular the recent events in Houla," his office said in a statement.
"He conveyed in frank terms his view to President Assad that the six-point plan cannot succeed without bold steps to stop the violence and release detainees, and stressed the importance of full implementation of the plan."
The Syrian government described the talks as "positive and constructive."
"Positive & constructive meeting between Annan & President Assad this morning. Details discussed to push forward the plan & end violence," foreign ministry spokesman Jihad Makdissi said in a posting on Twitter.
The Assad government has vowed to crush an Arab Spring-inspired uprising, with the crackdown triggering a low-level insurgency by some regime opponents who have taken up arms.
At least 108 people were killed in the central Syrian village of Taldu on Friday and Saturday, among them 49 children and 34 women, many shot dead at point-blank range.
The UN High Commissioner for Human Rights estimated on Tuesday that fewer than 20 of the 108 Houla victims were killed by artillery and tank fire.