A cessation of hostilities has officially come into effect in Syria.
The British-based Syrian Observatory for Human Rights said calm prevailed on most front lines after the cease-fire took effect at sunset on September 12.
Syria’s military says it is observing the truce, which does not apply to extremist groups.
The truce deal, brokered by Russia and the United States, is the second attempt this year by Washington and Moscow to end the five-year-long civil war.
Speaking hours after the cease-fire came into effect, U.S. Secretary of State John Kerry warned that it could be a last chance for peace in a united Syria.
Kerry said early reports indicated "some reduction in violence," but added that it was too early to draw a definitive conclusion about how effective the truce would be.
Kerry also said President Bashar al-Assad's forces are not supposed to bomb Syria's opposition any longer because of the truce. But he said the government could continue going after extremist militants in certain areas.
Earlier, Russia's Foreign Ministry expressed concern that some armed rebel groups had refused to abide by the truce.
If the truce holds for a week, the United States and Russia are to carry out coordinated air strikes against Jabhat Fateh al-Sham, formerly the Al-Qaeda-linked Al-Nusra Front, and Islamic State militants.
The conflict in Syria has left more than a quarter of a million people dead and more than 11 million others displaced, according to the UN.