UNITED NATIONS — The United Nations says it wants to rethink how humanitarian aid is delivered to the world’s spreading crises.

But as of Thursday, only about 90 of the 193 countries recognized by the United Nations had promised to attend the big event it is organizing to do just that: the World Humanitarian Summit. And the medical aid charity Doctors Without Borders announced that it would pull out, calling the conference a “fig leaf of good intentions” and saying that countries in conflict were ignoring “systemic violations” of international humanitarian law.

“Disappointing,” the spokesman for the United Nations, Stéphane Dujarric, said Thursday in response to the decision by Doctors Without Borders.

He said the summit meeting, scheduled to start on May 23 in Istanbul, was expected to address the issues that Doctors Without Borders regarded as priorities, including the obligation of warring parties to grant unimpeded access to humanitarian aid.