Some 1,500 boys have been verified as child soldiers in Yemen, but the actual number of children who have been drawn into the war is probably far higher, a UN spokeswoman said Tuesday.Most of the boys were recruited by Houthi rebels over the past three years, spokeswoman Ravina Shamdasani said at a press briefing in Geneva.Recruiters often entice the boys by promising them financial rewards or social status."Many are then quickly sent to the front lines of the conflict or tasked with manning checkpoints," Shamdasani said.All child soldiers in Yemen must be immediately released, the UN human rights office demanded.The war in Yemen killed 4,667 civilians and injured 8,180 others between March 2015 and February 23, according to the latest UN count.Meanwhile, the UN's aid chief had to cancel a planned visit to the south-western city of Taiz on Tuesday.Stephen O'Brien has been in Yemen since Sunday, visiting different cities."Despite all parties' assurances, we were denied access on frontline from Ibb to Taizz city today. Lack of access = more suffering," O'Brien wrote on his Twitter account.He did not give a reason, but according to the Al-Arabiya channel O'Brien's convoy was told "that their safety was the main concern."The Saudi-owned channel added that the Houthi rebels shelled the city to stop the visit.While visiting Aden city on Monday, he urged the warring parties to continue providing humanitarian access and called on the international community to step up its funding for life-saving operations.According to the UN, almost 19 million people in Yemen need humanitarian assistance.The conflict in Yemen started when the Houthis pushed towards the capital Sana'a in early 2014.In March 2015, they forced President Abd Rabu Mansour Hadi to flee, sparking a Saudi-led air campaign in support of government forces.