The UN says two aid convoys are stuck because of rows about security
The ceasefire in Syria has been extended by another 48 hours, the US State Department has said.
The decision, made by US Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov, comes two days after the start of the truce.
State Department spokesman Mark Toner said: "There was agreement that as a whole, despite sporadic reports of violence, the arrangement is holding, and violence is significantly lower.
"As part of the conversation they agreed to extend the cessation for another 48 hours."
Earlier this week, Mr Kerry said that the ceasefire could be the last chance to work towards peace in Syria.
The extension of the truce gives humanitarian agencies more time to access war-torn cities across Syria.
However the UN has said that continuing rows over security caused two aid convoys to get stuck in no-man's land.
The convoys, which were bound for Aleppo, each comprised of around 20 trucks containing mostly food and flour.
UN Secretary General Ban Ki-moon said he was in talks with Russia and the US to guarantee the security of aid convoys.
He said: "It's crucially important (that) the necessary security arrangements should be given so that they can be allowed to cross the lines.
"I have been urging the Russian government to make sure that they exercise influence on the Syrian government, and also the American side to make sure that Syrian armed groups, they also fully cooperate."
The UN Special Envoy for Syria, Staffan de Mistura, told a news briefing yesterday there had been "a significant drop in violence".
"Calm seems to have prevailed across Hama, Latakia, Aleppo City and rural Aleppo and Idlib – with only some allegations of sporadic and geographically isolated incidents."
Mr de Mistura also reported that sources on the ground in Aleppo city said the situation had "dramatically improved" with no airstrikes.
While the capital, Damascus, and central Syria also remained calm with "reports limited to some clashes around Harasta between government and opposition forces."
According to the UN Office for the Coordination of Humanitarian Affairs (OCHA), Syria is one of the most complex and dynamic humanitarian crises in the world today.
Since March 2011, more than a quarter of a million Syrians have been killed and over one million have been injured.
Some 4.8 million Syrians have been forced to leave the country, and 6.5 million are internally displaced, making Syria the largest displacement crisis globally.
In 2016, an estimated 13.5 million people - including six million children - are in need of humanitarian assistance. Of these 5.47 million people are in hard-to-reach areas, including close to 600,000 people in 18 besieged areas.