A fragile ceasefire has begun in Syria amid continued violence
The Syrian government and 97 rebel and militant groups said they would abide by the ceasefire.
The ceasefire excludes the ISIS and al-Qaida's branch in Syria, the Nusra Front.
But only a few hours after the ceasefire began, a Syrian rebel commander already said his fighters had registered numerous government ceasefire violations and warns they could lead to the collapse of the agreement.
The Syrian state-run news agency also reports that armed groups have fired several shells on residential areas in the capital in what it reports was the first breach of the ceasefire.
SANA says the shells were fired Saturday by "terrorist groups" entrenched in Jobar and Douma, both opposition-held suburbs of Damascus.
Lt. Col. Fares al-Bayoush, commander of the 1,300-strong Fursan al-Haq Brigade, a US-backed rebel faction, tells The Associated Press that his group and others affiliated with the mainstream Free Syrian Army are so far abiding by the truce.
He says continued government breaches however will force rebel factions to retaliate, although he adds that the ceasefire has sharply reduced government attacks across northern Syria where his group is based.
A top military official says Russia has grounded its warplanes in Syria to help secure the cease-fire. "The Russian Federation has completely stopped attacks in the 'green zone', that is to say those areas and armed units that have sent to us requests for cease-fire," Maj. Gen. Sergei Rudskoy said Saturday at a briefing in Moscow.
He said 17 opposition units have contacted the Russian military to adhere to the truce that became effective at midnight on Friday.
ISIS attacks throughout the country
ISIS claimed responsibility for a suicide truck bombing in central Syria, which killed at least two people. The blast rocked the area only hours after the start of the ceasefire engineered by Russia and the US.
The IS-affiliated Aamaq news agency says the suicide bomber targeted a military post near the town of Salamiyeh. Syrian state media said two people were killed in the blast, while an opposition group that monitors the conflict said three were killed.
ISIS (also known as the Islamic State, ISIL or Daesh) have also stormed a northern border town that was captured months ago by Kurdish fighters, according to a Syrian rebel official.
Talal Sillo, a spokesman for the predominantly Kurdish Syria Democratic Forces, said Saturday that the IS fighters have attacked the town of Tal Abyad. The fighting began after midnight Friday and was still ongoing, Sillo said. Tal Abyad that has been held by Kurdish fighters since July.
The Britain-based Syrian Observatory for Human Rights also confirmed the fighting in Tal Abyad.
Overall, opposition activists in different parts of Syria said the situation has been "cautiously calm" since the truce went into effect at midnight Friday.