U.N. envoy for Syria Staffan de Mistura is in Syria where he is due to hold talks Tuesday with officials on implementing a cease-fire this week.
He arrived late Monday in the Syrian capital, Damascus, for the unannounced visit.
World powers agreed last week to a cessation of hostilities, set to begin Friday, in order to let badly needed humanitarian aid be delivered.
The cease-fire is meant to help make it possible to resume international peace talks that collapsed earlier this month in Geneva after just three days.
Since then, the violence has shown no signs of letting up. The Syrian government has continued an offensive backed by Russian airstrikes. A wide array of rebel groups also continue to fight in the country, each with different objectives and international backers.
World powers have said they hope to resume peace talks by the end of the month.