The United Nations now estimates there are five million Syrians in need of humanitarian aid living in hard-to-reach areas, an increase of nearly a million people over the previous estimate.

Stephen O'Brien, the U.N.'s emergency relief coordinator said the increase of 900,000 people is mainly due to the inclusion of areas in parts of Aleppo, Raqqa and Hasakeh as a result of insecurity and constrained humanitarian access.

O'Brien also flagged continuing attacks on hospitals and medical facilities as violations of international law and Security Council resolutions.

He said that Physicians for Human Rights documented 365 attacks on 259 medical facilities as of late April, with 76 percent of attacks attributed to Syrian government forces. He said the U.N. has received reports of many more attacks on medical facilities since then.