The Syrian government in 2015 ignored most United Nations requests to deliver humanitarian aid to some of the 4.6 million people in hard-to-reach and besieged areas, and only 620,000 received any aid at all, the U.N. humanitarian air chief, Stephen O'Brien, said on Wednesday.O'Brien told the U.N. Security Council that the United Nations made 113 requests to the Syrian government last year for the approval of inter-agency aid convoys, but only 10 percent were able to deliver assistance. Another 10 percent were approved in principle by the Syrian government, but could not proceed due to a lack of final approval, lack of security or no deal on safe passage, while the U.N. put 3 percent on hold due to security concerns. O'Brien added that the remaining 75 percent of requests went unanswered. "Such inaction is simply unacceptable," he said. "The impact on the ground is tangible: In 2013, we reached some 2.9 million people through the inter-agency convoy mechanism, but only 620,000 [in 2015]. More and more people are slipping out of our reach every day as the conflict intensifies and battle lines tighten," O'Brien said.The U.N. said 13.5 million people in Syria need humanitarian aid, up from 1.3 million in 2014. "Even with the worsening situation and continued access challenges, humanitarian workers in Syria continue to stay and deliver aid often at great personal risk," O'Brien said. He said in 2015, food was delivered to nearly 6 million people a month; health aid to almost 16 million people; water, sanitation and hygiene support to 6.7 million; and basic household items to 4.8 million. "Let me be clear: The continued suffering of the people in Syria cannot be blamed on humanitarian organizations and staff," O'Brien said. "It is the failure of both the parties and the international community that have allowed this conflict to continue for far too long." Syria U.N. mediator Staffan de Mistura hopes to convene talks on Friday on ending the civil war, but those plans appeared doubtful after the opposition said it would not show up unless attacks on civilian areas stopped first.Meanwhile, the U.N. said the number of besieged areas in Syria's conflict has risen to 18, up from 15 earlier this month, with as many as half-a-million people now affected. O'Brien on Wednesday called on Syria's government to allow sustained access to besieged areas and to the estimated 4.5 million people in hard-to-reach areas.Officials said the rare convoys that reached a few besieged communities earlier this month, after images of emaciated Syrian children were widely shared online, are not enough and that the food delivered will soon run out. "One-off access … is not the kind of access we need to prevent starvation," O'Brien told reporters. He also called for immediate medical evacuations from besieged areas for the sick and wounded. O'Brien called the idea of airdropping aid to besieged areas "risky" and insufficient, but diplomats said all options are still being discussed. World Food Programme (WFP) Executive Director Ertharin Cousin told reporters that for airdrops to happen, her agency would need secure airspace, assurances that the aid gets to the most vulnerable and enough space on the ground to safely drop the "large tonnages" necessary to be meaningful food aid.The U.N. said food aid reached less than 1 percent of people in besieged areas last year. About 181,000 are besieged by Syria's government, and about 200,000 are besieged by DAESH. Cousin said it is "just a matter of time" before the world again sees the kind of images of suffering Syrians that prompted international outrage and the rare aid convoys earlier this month. While the U.N. says all sides in the conflict have blocked the delivery of aid, it has repeatedly criticized Syria's government. U.N. Secretary-General Ban Ki-moon's latest report on the crisis said since the beginning of 2015, just 13 inter-agency convoys have been approved by the government and completed, out of 113 requested. "We cannot for one minute think the situation is improving," the U.S. ambassador to the U.N., Samantha Power, told reporters.The civil war was sparked by a Syrian government crackdown on a pro-democracy movement in early 2011 has claimed more than 250,000 lives according to U.N. figures, which also displaced nearly 10 million people. The vast majority of Syrian refugees have fled to Lebanon, Jordan, Iraq and Turkey.