The "UN" cannot invade anywhere as there is no "UN army". UN missions are undertaken by indivdual states, or a collective of states, on behalf of, and with the permission of, the UN.
Unfortunately, or fortunatley, it is the responsibility of the Security Council to decide whether military intervention in a conflict is acceptable. This means the permanent members - UK, US, France, Russia, China, who all have a veto - can prevent action should they disagree. If one of the 5 do not want action taken against Burma, then no action can be sanctioned (which is how America can stop any action UN against Israel, Russia can stop any UN action against Serbia, China can stop any UN action against Sudan, etc, etc)