Fighting has eased in parts of Syria where a Russian-led initiative to halt the country's six-year war took effect from midnight, activists say.
The deal unveiled by Russia on Thursday to set up four "de-escalation zones" was backed by Turkey and Iran, but the main Syrian opposition grouping says it has little faith in it.
Under the plan, the four zones will be established in:
- Rebel-held areas in the north-western province of Idlib and adjoining districts of Latakia, Aleppo and Hama
- Parts of Homs province in the centre, where rebels hold a stretch of territory
- The opposition enclave of Eastern Ghouta, near the capital Damascus
- Daraa and Quneitra provinces, in southern Syria, where rebels have a large presence
Meanwhile the UK-based Syrian Observatory for Human Rights (SOHR) said Syrian fighter jets had bombed rebel positions in Hama on Saturday.
A rebel commander confirmed that fighting had broken out after midnight, Reuters reports.
There were also reports of shelling and gunfire in other parts.
The opposition High Negotiations Committee (HNC), said on Friday the safe zones plan lacked "safeguards and compliance mechanisms". It also said it did not accept Iran as a guarantor of the deal.
UN special envoy for Syria Staffan de Mistura said the agreement was important because it was being implemented by three countries who could make it work.
The Syrian military have so far not commented on the issue.
The so-called "memorandum on the creation of de-escalation areas" was announced by the Russian military after talks in Kazakhstan's capital Astana.
Russia's foreign ministry published the full text of the agreement on Saturday.
The objective is to halt attacks by all sides and to "provide the conditions for the safe and voluntary return of refugees" in addition to the speedy provision of relief supplies and medical aid.
The four zones will remain in place for six months, but their borders have to be finalised by Russia, Iran and Turkey by 4 June.
Russian Deputy Defence Minister Alexander Fomin said on Friday there had been no bombing raids by Russian aviation in the four zones since 1 May.
But he stressed Russia's air force would continue striking militants from the Islamic State (IS) group elsewhere in Syria.
The US expressed concern over Iran, saying the country had "only contributed to the violence, not stopped it".
The Syrian government of President Bashar al-Assad is not a signatory, but its state news agency said it supported the plan.
The Astana talks were meant to shore up an oft-violated ceasefire originally agreed in December.
Syria's war has left more than 300,000 people dead since it erupted in 2011.