Activists say Syrian troops resumed their shelling of residential neighborhoods dominated by rebels in the central city of Homs on Sunday. Several people died.
The country restricts access of foreigners, making it difficult to verify reports of violence.
If the cease fire holds, the U.N. hopes to send a full team of 250 observers to Syria.
Since the cease-fire formally took effect Thursday, the military has halted shelling and mortar attacks in residential areas held by rebel forces - except in Homs. Before the truce, shellings were the daily norm in many areas of Syria.
The reported shelling in Homs was threatening the truce as U.N. observers were en route to Damascus on Sunday. Each side in the Syria conflict has accused each other of truce violations.
Western powers and opposition leaders remain skeptical about Assad's willingness to ease his tight grip on the country, ruled by his family for four decades. The regime appears to have complied with parts of the truce plan, while flouting others.
The Associated Press contributed to this report.