An unprecedented summit to revamp humanitarian aid and global responses to modern-day crises has opened in Turkey.

The first World Humanitarian Summit is being convened in Istanbul on Monday and Tuesday in a bid to better tackle what the United Nations describes as the worst humanitarian crisis since World War II.

The gathering was conceived four years ago by U.N. Secretary General Ban Ki-moon. In preparation, 23,000 people were consulted in over 150 countries, U.N. officials say.

The U.N. under-secretary general for humanitarian affairs and emergency relief coordinator, Stephen O'Brien, said the summit's success would be defined not by funds raised but by the number of commitments to action made.

He said the summit is "a once in a generation opportunity to set in motion an ambitious and far-reaching agenda."