Details of of the plan must still be worked out over the next several weeks. There were limited reports of bombing in northern Homs and Hama, and the southern province of Daraa, areas expected to be part of the "de-escalation zones," activists said. There were no immediate reports of casualties.
It is not clear how the cease-fire 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.
Syria's government has said that although it will abide by the agreement, it would continue fighting "terrorism" wherever it exists, parlance for most armed rebel groups fighting government troops.
The armed opposition delegation to the talks in the Kazakh capital of Astana said in a statement released early Saturday that the truce should include all Syria and not just specific areas. It said some maps of the "de-escalation zones" that were released are not accurate and will not be accepted because the armed opposition did not negotiate them.
Still, opposition activists in southern, central and northern Syria told The Associated Press on Saturday the situation is by far much better than previous days, with no airstrikes reported.
The government-controlled Syrian Central Military Media reported there was "relative clam" Saturday morning in the "de-escalation zones" nine hours after the deal went into effect.
The Britain-based Syrian Observatory for Human Rights, which has activists around the country, said the government's helicopter gunships dropped three barrel bombs on the rebel-held Latamneh area in central Syria where fighting was reported between rebels and troops. It added that government forces shelled rebel-held neighborhoods of the capital Damascus.
"Despite some violations the situation is much calmer than before," said opposition activist Mohammed al-Homsi, speaking via Skype from northern Syria.
Ahmad al-Masalmeh, who is based in the southern province of Daraa that borders Jordan, said there were six breaches in the province when government forces shelled opposition-held areas.
Al-Homsi, al-Masalmeh and opposition activist Osama Abu Zeid said government warplanes have not carried out any airstrikes on rebel-held areas since Wednesday, a day before the deal to set up the zones was signed in Kazakhstan.
Syrian, Russian, Turkish and U.S.-led coalition aircraft sometimes operate in the same areas in Syria. It is not yet clear how the new plan would affect flightpaths of coalition warplanes battling Islamic State group militants and other radical groups — and whether the American air force would abide by a diminished air space.
The Pentagon said the de-escalation agreement would not affect the U.S.-led air campaign against IS.
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 fueling 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, drawing the ire of Assad and his government.
Yet troops from the three countries are now expected to secure four safe zones. An official with Russia's military general staff said other countries may eventually have a role in enforcing the de-escalation areas.
Russian Col.-Gen. Sergei Rudskoi told reporters on Friday that personnel from Russia, Iran and Turkey will operate checkpoints and observation posts.
But it's difficult to imagine how many boots on the ground would be needed to monitor the yet to be mapped areas or how and where exactly Russian, Iranian and Turkish troops would patrol.
A previous cease-fire agreement that went into effect on Dec. 30 helped reduce overall violence in Syria for several weeks but eventually collapsed. Other attempts at a cease-fire in Syria have all ended in failure.