Syrian government forces and rebels clashed in the north-western province of Hama on Friday shortly after a Russian-led deal to establish safe zones took effect, a monitor and a rebel official said.
The deal on de-escalation zones was signed during the fourth round of the Syria peace talks in the Kazakh capital, Astana, on Thursday. Points to monitor the ceasefire will be also be arranged there, it indicated.
The Kazakhstan agreement calls for setting up four zones in northern, central and southern Syria.
"Psychologically, residents are relieved", he said.
The UK-based Syrian Observatory for Human Rights (SOHR) said Syrian fighter jets on Saturday bombed rebel positions in Hama.
"Despite these violations, we can still say that hostilities have dropped", said Observatory head Rami Abdel Rahman.
Russia's military says the agreement setting up four de-escalation zones in Syria will go into effect at midnight.
It said the agreement "lacks the minimal elements of legitimacy" and also rejected any role for government ally Iran as a guarantor of the deal. The wide array of rebel groups include some supported by Turkey, the United States and Gulf monarchies. The UN did not broker the deal but has expressed its support.
The agreement would initially last six months but could be extended by the guarantors.
A day after the USA gave tepid support for the establishment of Syrian safe zones to try to separate combatant forces and reduce the amount of fighting, they appear to have noticed that this might get in their way of their own attacks across the country, and are pledging to ignore the demilitarization of the zones.
Russian Federation says maps delineating the zones should be ready by June 4. Russia's Defense Ministry said earlier that the fight against Islamic State and Jabhat al-Nusra terrorist groups (outlawed in Russia - TASS) would be continued inside and around those zones.
In Idlib province in particular, Fateh al Sham is a leading and powerful component of the opposition forces controlling the region, and a key ally for other rebel groups against the regime.
The group will then "take steps to complete by June 4, 2017, the preparation of the maps of the de-escalation areas and security zones and to separate the armed opposition groups from the terrorist groups".
Activist Ibrahim al-Idlibi, who is based in Idlib, said the situation was calm in the province, which has been the target of daily raids by regime and Russian planes for the past few months.
"We will send this list to the Russians via the Turks", the source said, speaking on condition of anonymity.