Reports from Syria say violent clashes broke out less than two hours after the start of a nationwide cease-fire that began at midnight Friday, local time.
The deal includes the Syrian government and several rebel groups, but not certain Islamist groups. The monitoring group Syrian Observatory for Human Rights, based in Britain, said Friday that rebels violated the cease-fire by taking over territory in the province of Hama.
But a spokesman for the Jaish al-Nasr rebel group, told Reuters news service that government forces violated the deal by shelling areas in Idlib province, which borders Hama.
Russia, which backs Syrian President Bashar al-Assad, and Turkey, which supports rebel groups, have said they would guarantee the truce, which came after Turkey, Syria and Russia last week said they were ready to broker a peace deal in the six-year war.
There are dozens of groups fighting in Syria, and it was not immediately clear which of them have agreed to halt hostilities.
There have been three previous attempts to secure a nationwide cease-fire in Syria. The most recent was in September, but it lasted only a little over a week.