Two people were killed and two wounded in a suicide bombing in a government-held town in central Syria, hours after a truce was implemented.
Despite the bombing in Salamiyeh on February 27, reports suggested the cease-fire had brought relative calm to parts of Syria.
The U.S.-Russian accord was accepted by President Bashar al-Assad's government and many of his enemies.
Under it, fighting should cease so aid can reach civilians and talks can open to end a war that has killed more than 250,000 people and made 11 million homeless.
The truce does not include either the Islamic State group or Al-Qaeda's branch in Syria, known as the Nusra Front.
In Moscow, military officials said Russia had suspended air strikes in a "green zone" in Syria in line with the accord.
Opposition activists in different parts of Syria said the situation has been "cautiously calm" since the truce went into effect at midnight, reporting sporadic violations.