The United Nations humanitarian chief says a "humanitarian catastrophe" is unfolding in Yemen.

Stephen O'Brien says the situation is exacerbated by increasing restrictions on efforts to respond to the staggering needs of millions of people, including the diversion of a U.N. aid ship by Saudi-led coalition forces.

Stephen O'Brien painted a grim picture Tuesday of the war-ravaged country: more than 35,000 casualties since March 2015 including over 6,000 deaths; at least 7.6 million people "severely food insecure"; and more than 3.4 million children now out of school.

O'Brien's briefing to the U.N. Security Council, requested by Russia, was the first focusing on the humanitarian crisis sparked by the country's civil war.

He urged the council to demand that all combatants facilitate unconditional humanitarian access to all parts of Yemen.