The United Nations humanitarian office says it needs $1.8 billion dollars to provide critical and life-saving assistance to 13.6 million people who have been affected by the escalating conflict in Yemen.Four out of five Yemenis are in need of humanitarian assistance, and increased attacks on essential civilian and economic infrastructure have deprived millions of families of their livelihoods, the Office for the Coordination of Humanitarian Affairs in Geneva said Thursday as it launched its appeal.In January, the U.N. group delivered food rations for 2.6 million people and water deliveries to more than 234,000 people.