UN humanitarian coordinator Yacoub El Hillo said the world body hoped to take advantage of the first major truce in five years of conflict to distribute supplies to an extra 154,000 people living in besieged areas over the next five days.
A successful truce would also create a more favourable backdrop for peace talks that collapsed in acrimony in early February as a Russia-backed regime offensive in northern Syria caused tens of thousands to flee.
The main opposition grouping yesterday described the ceasefire as "positive" but lodged a formal complaint with the United Nations and foreign governments about breaches.
"We have violations here and there, but in general it is a lot better than before and people are comfortable," said Salem al-Meslet, spokesman for the Saudi-backed High Negotiations Committee.
Meslet said the opposition would like to see the truce "last forever" and that it was the "responsibility of the United States to stop any violations".
It said the breaches had killed 29 people and wounded dozens.
The HNC has said it did not receive any maps of areas included in the ceasefire or documents explaining the monitoring mechanism.
Syria's Al-Watan newspaper, which is close to the government, said yesterday that those maps were still being "kept secret".
The ceasefire does not apply to territory held by the Islamic State group and Al-Qaeda affiliate Al-Nusra Front.
After several days of deadly clashes, the army succeeded in reopening it today, the Syrian Observatory for Human Rights said.
At least 26 pro-government fighters and 14 IS jihadists were killed in the fighting around the town of Khanasser, the Britain-based monitoring group said yesterday.