Syrian regime attacked rebel group in northwest Syria and dropped two barrel bombs on opposition areas on early Saturday after called a breach of the cessation of hostilities plan that came into effect at midnight.
The rebel group said Assad regime attacked its position at 4am (0200 GMT).
During the attack, three fighters from the rebel First Coastal Division were killed while repulsing the attack in the Jabal Turkman region close to the Turkish border in Latakia province, Fadi Ahmad, the group's spokesman said.
"It is a violation (of the agreement)," Ahmad said, identifying the attack as a ground assault without air strikes. "Currently, the regime has halted the attack."
Fares Bayoush, head of the Fursan al Haqq rebel group, also said, "There are areas where the bombardment has stopped but there are areas where there are violations by the regime such as Kafr Zeita in Hama, via targeting with artillery, and likewise in Morek in northern Hama countryside."
He added if the violations carry on, the deal could collapse.
In the same day, Assad forces also dropped two barrel bombs and opened fire in the Eastern Ghouta near Damascus.
The Syrian regime had stated that it will respect the deal prepared by the United States and Russia.
Jaish al Islam spokesman Islam Alloush indicated that in one incident regime forces attempting to advance "were dealt with machine guns".
The regime had no immediate comment related to the incident.
US-Russian agreement on a cessation of hostilities has started to take effect at Saturday midnight local time (2200GMT) on Feb. 27
The deal does not include DAESH, the Al Qaeda-linked Nusra Front and any other militias designated as terrorist group by the UN Security Council. Both the US and Russia continue to target those terrorist groups with air strikes in Syria.
UN's Syria mediator Staffan de Mistura said on Friday that he intends to reconvene Syrian peace talks on March 7 if the cessation of hostilities largely holds and allows for greater delivery of humanitarian relief.
According to the Syrian Center for Policy Research, 470,000 Syrians have been killed and 1,900,000 others have been injured while the Syrian Observatory for Human Rights (SOHR) said the number of dead in Syrian civil war is estimated to be more than 370,000 between March 18, 2011 and Feb. 22, 2016.
At least 11 million have fled their homes and a great majority of them have been living neighboring countries such as Turkey, Jordan and Lebanon.
More than a million refugees have reached Europe, while Turkey hosts the most in the world, opening its doors to 2,7 million Syrian fleeing civil war.