The U.N. chief says the "onus is on Syria" to keep the government's promise and hold to a cease-fire.
Ban Ki-moon says that "as of this moment, the situation looks calmer" on the ground in Syria but that the cease-fire Thursday is "very fragile" and may be broken anytime.
He tells reporters that it may take just a gunshot to derail peace.
Ban told reporters Thursday in Geneva that "the world is watching, however, with skeptical eyes, since many promises previously made by the government of Syria have not been kept."
He says the government must that its "words will be matched by their deeds."
A fragile cease-fire brokered by the U.N. took hold in Syria on Thursday.
Regime forces apparently halted widespread attacks on the opposition.