Geneva: Some Syrians in the besieged areas of Daraya and Deir Al Zor have been reduced to eating grass because food supplies are cut off, the UN World Food Programme said on Friday.
"In the most severe cases, they are enduring entire days without eating, sending children to beg and eating grass/wild vegetation," a report said.
Deir Al Zor is under seige by IS forces, while Daraya is besieged by government forces and has become a focus of UN efforts to get aid to all of Syria. Syria's government has not yet granted permission for aid to go to the city.
Households in the two cities were unable to eat more than one meal per day and giving priority to children, said the WFP report, a survey of Syrian food market conditions in February.
Fresh bread was "sporadically available at an extortionate cost" in Daraya, 30 times above the market price in nearby Damascus. Rice was 17 times higher than Damascus prices.
Despite a widespread truce that has lasted almost three weeks, Syria's government has refused to give permission for UN aid convoys to enter six areas under siege by its forces, including Daraya.
UN humanitarian advisor Jan Egeland said on Thursday that countries backing the Syrian peace talks had given the Syrian government seven days to answer a UN request to deliver aid.
"It is in violation of international law to prevent us from going," he said, adding that the six areas were no more strategic or symbolic than other areas that had already received aid convoys.
"In Daraya there has been fighting, but we had a very clear impression that we will not be having any problems in delivering if we get the two sides to agree to the cessation of hostilities so that we can deliver to the few thousand people there, civilians who are in a very, very difficult position," Egeland said.
With no hope of getting convoys into Deir Al Zor by road, the UN hopes to do air drops of food. But a first attempt failed because the plane had to fly so high and fast to avoid the threat of surface-to-air missiles, causing the parachutes to fail because of the severe jolt when they opened.
Meanwhile, Russian Foreign Minister Sergei Lavrov discussed in a phone call with Iranian Foreign Minister Mohammad Javad Zarif the implementation of ceasefire in Syria, the Russian Foreign Ministry said in a statement on Friday.
"The need for a stable political process with the participation of representatives of the Syrian government and a wide range of opposition groups.... was stressed," the ministry said in a statement posted on its website.
UN mediator Staffan de Mistura after a fifth day of peace talks said on Friday that Syria's government must do more to present its ideas about a political transition and not merely talk about principles of the peace process.
"We are in a hurry," de Mistura told reporters after an "intense" day and meetings with Syria's government delegation and the main opposition, the High Negotiations Committee.
De Mistura said he had given both sides homework to do so that the negotiations could go faster on Monday, and during the second week of talks he would try to build a "minimum common platform" for a better understanding on the political transition.
Meanwhile, Syrian rebel factions on Friday condemned a declaration of federalism in Kurdish-controlled regions of northern Syria and vowed to resist it by force, a day after those areas voted to seek autonomy.
A statement from a number of Syrian insurgent groups, some of whom are represented in the main opposition body that is participating in peace talks, said the federalism announcement was a "project to divide" Syria.
Syria's Kurdish-controlled northern regions voted on Thursday to seek autonomy under a federal system, drawing rebukes from the main opposition's High Negotiations Committee, the Damascus government, Turkey and Washington.
The rebel statement said this was "exploitation" of the Syrian uprising that began five years ago and descended into civil war, and condemned what it said were attempts by "groups... which took control of parts of Syrian land to establish their racial, nationalist and sectarian entities".
It compared Kurdish groups to IS, and said the YPG militia and its political arm the PYD were terrorists.
The YPG, which has been backed by Washington in its fight against IS, has beaten back the militants to control swathes of northern Syria, but the PYD has so far been excluded from peace talks that began this week in Geneva.