ISTANBUL — An ambitious two-day summit to revamp humanitarian aid and global responses to modern-day crises opened in Turkey on Monday with lofty goals overshadowed by concerns that key participants are violating refugee rights and humanitarian law.

The World Humanitarian Summit was convened in an attempt to tackle what the UN calls the worst humanitarian crisis since World War II.

An estimated 125 million people worldwide require humanitarian assistance, among them 60 million displaced by conflict, natural disasters, and climate change. The gathering was conceived by UN Secretary General Ban Ki-moon.

Doctors Without Borders pulled out of the event, calling it “a fig-leaf of good intentions’’ at a time when ‘‘shocking violations of international humanitarian law and refugee rights’’ go unchecked.

Associated Press