Syrian rebels in the country's northwest said they came under attack from government ground forces at 4 a.m. (0200 GMT) on Saturday in they called a breach of the cessation of hostilities plan that came into effect at midnight.
Three fighters from the rebel Second Coastal Division were killed while repelling the attack in the Jabal Turkman area near the Turkish border in Latakia province, Fadi Ahmad, spokesman for the affiliated First Coastal Division, told Reuters.
The Syrian military could not immediately be reached for comment. The Syrian government has said it will respect the agreement drawn up by Russia and the United States, but that it will continue to fight the al Qaeda-linked Nusra Front and Islamic State which are not covered by the deal.
"It is a violation (of the agreement)," Ahmad said, describing the attack as a ground assault with no air strikes. "Currently, the regime has halted the attack," he added.
The First and Second Coastal Divisions are part of a loose alliance of rebel groups known as the Free Syrian Army.
Northwestern Syria is one of the areas where the Nusra Front operates near rebel groups that have approved the cessation of hostilities. Ahmad said the positions attacked on Saturday are controlled by his group, and the Nusra Front has no presence there. Nusra has called for an escalation of attacks.
The Britain-based Syrian Observatory for Human Rights said there was calm in many parts of the country shortly after midnight when the agreement came into effect.