The deal to create de-escalation zones in the major areas of conflict in western Syria took effect at midnight and is expected to remain valid for at least six months.
The agreement on de-escalation zones in Syria will be valid for at least six months, said the Russian foreign ministry on Saturday.
Russia, Turkey and Iran agreed in a memorandum signed on May 4 to establish four separate de-escalation zones in Syria after rounds of talks in Kazakhstan capital Astana.
The largest de-escalation zone includes Idlib province and the adjoining districts of Hama, Aleppo and Latakia provinces. The other three zones are in northern Homs province, the Eastern Ghouta region east of the capital Damascus and along the Jordanian border in southern Syria.
The Syrian Observatory for Human Rights and a spokesman for the Jaish al-Nasr rebel group said the regime forces clashed with rebels in the northwestern Syrian province of Hama.
The Britain-based war monitoring group said the warring sides exchanged shelling and were fighting in a rebel-held village and nearby areas of the Hama countryside.
The Observatory said it had not recorded any deaths as a result of fighting in the four zones since midnight, but there had been some violations.
TRT World's Abubakr al Shamahi brings the latest.
Mohammed Rasheed, a spokesman for the Jaish al Nasr rebel group based in Hama, confirmed that fighting had broken out after midnight.
Rasheed said rebel-held Idlib province to the north of Hama was almost completely quiet, but the attacks, which included barrel bombs, were focused on the northern Hama frontline area.
"The bombardment has not stopped, it is no different from before," he said.