A deal hammered out by Russia, Turkey and Iran to set up "de-escalation zones" in mostly opposition-held parts of Syria has gone into effect.
The plan is the latest international attempt to reduce violence in the war-torn country and the first to envisage armed foreign monitors on the ground in Syria.
The United States is not party to the agreement and the Syrian rivals have not signed up to the deal. But the armed opposition was highly critical of the proposal, saying it lacked legitimacy.
The plan, details of which will still be worked out over the next several weeks, went into effect at midnight on Friday.
There were limited reports of bombing in northern Homs and Hama, two areas expected to be part of the zones, activists said.
It is not clear how the ceasefire or de-escalation zones will be enforced in areas still to be determined in maps to emerge a month from now.
Russian officials said it will be at least another month until the details are worked out and the safe areas established.
In the tangled mess that constitutes Syria's battlefields, there is much that can go wrong with the plan, agreed on in talks Thursday in Kazakhstan.
There is no clear mechanism to resolve conflict and violations like most other previous deals struck by backers of the warring sides.
A potential complication to implementing the plan is the crowded airspace over Syria. The deal calls for all aircraft to be banned from flying over the safe zones.
Syrian, Russian, Turkish and US-led coalition aircraft operate in different, sometimes same areas in Syria.
It is not yet clear how the new plan would affect flightpaths of US-led coalition warplanes battling Islamic State militants and other radical groups - and whether the American air force would abide by a diminished air space.
Russia and Iran, two of the plan's three sponsors, are key allies of President Bashar Assad's government and both are viewed as foreign occupation forces by his opponents.
Rebels fighting to topple Assad are enraged by Iran's role in the deal and blame the Shiite power for fuelling the sectarian nature of Syria's conflict, now in its seventh year.
Turkey, the third sponsor, is a major backer of opposition factions and has also sent troops into northern Syria, angering Assad and his government.
An official with Russia's military general staff said other countries may eventually have a role in enforcing the de-escalation areas.
Colonel General Sergei Rudskoi said Friday staff from Russia, Iran and Turkey would operate checkpoints and observation posts.
He said "security belts" would be created along the borders of the zones to prevent incidents and fighting between opposing sides.
The checkpoints and observation posts would ensure free movement of unarmed civilians and humanitarian aid and allow economic activities.
Rebels have expressed concerns the deal is a prelude to a de facto partitioning of Syria into spheres of influence.
Osama Abo Zayd, a spokesman for the Syrian military factions at the Kazakhstan talks, said it was "incomprehensible" for Iran to act as a guarantor of the deal.
A ceasefire was unsustainable in the presence of the Iranian-backed militias in Syria, he said, adding: "We can't imagine Iran playing a role of peace."
The US sent a senior White House official to the Kazakh capital of Astana, where representatives of Russia, Turkey and Iran signed the agreement, but had no role in the deal.
The idea of armed monitors is a new element. Observers deployed in the early years of the Syrian conflict, including United Nations and Arab League observers, were unarmed.
Western and Saudi-backed Syrian opposition coalition the High Negotiations Committee criticised the deal, saying it lacked legitimacy and sought to divide the country.
The HNC also said the deal was an attempt to give Syrian government troops military victories they could not achieve on the battleground by neutralising rebel-held areas and called on the US and other Arab allied countries to prevent the implementation of the deal.
A previous ceasefire agreement signed in Astana on December 30 helped reduce overall violence in Syria for several weeks but eventually collapsed and other attempts have ended in failure.
The safe zones will be closed to military aircraft from the US-led coalition, the Russian official who signed the agreement, Alexander Lavrentye, said.
Under the plan, Assad's air force, and presumably the Russians, too, would also halt flights over those areas.
In rebel-held Idlib, a protest was held on Friday against the plan, said to be a plot to "divide Syria".
"Any person or state who enters this land to divide it is the enemy of the Syrian people," activist Abed al-Basset Sarout told the crowd.
The Pentagon said the de-escalation agreement would not affect the US-led air campaign against IS.
"The coalition will continue to target Isis wherever they operate to ensure they have no sanctuary," said Pentagon spokesman Marine Major Adrian Rankine-Galloway.