Terje Roed-Larsen, who represents UN Secretary-General Kofi Annan in talks with the Lebanese and Syrian officials, said he had been concerned about the potential for violence in Lebanon both before and after the assassination of former prime minister Rafiq al-Hariri.
"I was and I continue to be deeply worried about the potential for violence in Lebanon," Roed-Larsen said.
"This is why I am urging all parties concerned to cool down their tempers."
Shortly after he expressed his fears, a car bomb exploded in Beirut early on Saturday wounding at least 10 people.
Asked if he was concerned about an assassination of another senior Lebanese figure, Roed-Larsen replied: "Yes, I am indeed."
He said the Lebanese should form a transitional government, continue investigating the killing of al-Hariri and prepare for free and fair elections in May.
Roed-Larsen said he expected Syria to withdraw all its troops from Lebanon before elections in May. He will travel to Beirut in the first week of April to verify the withdrawal for a 19 April report to the Security Council on Syrian compliance.
A joint Lebanese-Syrian military commission is to meet on 7 April to set a timetable for the withdrawal of Syrian forces.