All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
It is permissible to work for the UN in extending humanitarian aid since this relieves people in distress and alleviates their suffering. In this regard, Prophet Mohammad (PBUH) said: "There is a reward for serving any animate" {Related by Bukhari and Muslim}.
In fact, Islam recommends taking up such noble jobs, be the targeted groups Muslim or non-Muslim since Allah, The Almighty, Said (What means): "And they feed, for the love of God, the indigent, the orphan, and the captive, (Saying): "We feed you for the sake of God alone: no reward do we desire from you, nor thanks." {Al-Ensan, 8-9}. Moreover, The Prophet (PBUH) said: “The most beloved people to Allah are those who are most beneficial to the people. The most beloved deed to Allah is to make a Muslim happy, or to remove one of his troubles, or to forgive his debt, or to feed his hunger." {Related by Tabarani}.
Doing this noble job with the intention of serving and helping people entitles a person to a great reward from Allah, The Almighty, Who Said (What means): " Help ye one another in righteousness and piety, but help ye not one another in sin and rancour: fear Allah. for Allah is strict in punishment." {Al-Mai`dah, 2} .And Allah Knows Best.