The United Nations said medical evacuations from eastern Aleppo had not begun on Friday as it had hoped, as a lack of security guarantees and agreement. The U.N. wants to use the four-day pause to evacuate hundreds of sick and wounded from the besieged part of the city and to make food and aid deliveries, U.N. humanitarian spokesman Jens Laerke told a regular U.N. briefing. "It is an astronomically difficult operation whereby we need the security assurances from all sides to be there and to be adhered to, plus facilitation from all parties," he said.He declined to give details of the hold-up but he said the U.N. remained optimistic and was trying hard to unblock access. "It's not even day-by-day, it's hour-by-hour," he said. Russia announced a halt to its air strikes from Tuesday and the unilateral ceasefire from Thursday. Russia has told the U.N. it plans to halt bombing for 11 hours per day and the U.N. has asked for the period to be extended. Aid rations will run out by the end of this month, and fuel and medicine are also critically low, the U.N. has said.No aid has entered Aleppo since July 7 and food rations will run out by the end of October, UN chief Ban Ki-moon warned on Thursday. East Aleppo, which the opposition fighters captured in 2012, has been under siege by the army since mid-July and has faced devastating bombardment by the Bashar al-Assad forces and Russia since the launch of an offensive to retake the whole city on September 22. Nearly 500 people have been killed, more than a quarter of them children, since the assault began.More than 2,000 civilians have been wounded. The UN has been criticized by the opposition groups for focusing more on enabling people to leave than providing relief supplies to allow them to stay. A joint statement by the Syrian National Coalition and the Free Syrian Army said UN policy "plays into the Assad regime's plans to empty Aleppo."