It is hoped that emergency aid deliveries will start on Wednesday.
Russian forces are monitoring the ceasefire along a key access road in northern Syria that leads to rebel-held eastern Aleppo, reports say.
Some 250,000 civilians are trapped there.
UN Special Envoy for Syria Staffan de Mistura said if the truce held, aid deliveries "should be taking place very, very soon".
He said the UN was waiting for the Syrian government to authorise deliveries and he hoped the people of Syria could look forward to "no bombs and more trucks".