No legal obligation, but a moral one, surely. Does no-one have a duty to look after the poor? It's not their fault they are poor, any more than it's ours that we are rich - and by comparison, we are all loaded. We might think we earned our fortunes, but, by and large, we were born into it - or at least, born in a wealthy country where comfortable living is taken for granted.
I propose the wealthiest nation in the world should adopt the poorest nation in the world and share both nations' wealth equally; the 2nd wealthiest adopt the 2nd poorest and so on. And when that cycle's completed, if there is still a huge disparity, to do the same thing over again.
But it's just a dream ...