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 help, the UN aid chief said on Wednesday.Stephen O'Brien told the UN Security Council that last year the United Nations made 113 requests to the Syrian government for 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, insecurity or no deal on safe passage, while the UN put 3 percent on hold due to insecurity.O'Brien said 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. In total, the UN said 13.5 million people in Syria need humanitarian aid, up from 1.3 million from 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 that 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."