Humanitarian "access is essential if we are to continue saving millions of lives across Yemen," the Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator warned in his briefing to the Security Council on Tuesday.
"In the south, humanitarian agencies have to wait too long to get projects approved.
"Last year, we estimate that about 30 percent of NGO projects were not taken forward because the Government failed to agree them," Mark Lowcock added.
"We also remain concerned over proposed Government regulations that would hinder humanitarian agencies ability to move around the country as necessary in order to meet urgent needs.
"This past week the Government returned eight trucks, with medical supplies, held by Government forces in Marib since 30 January. However, after initial inspection, it appears that up to 70% of the supplies were missing," the UN official complained.
"But it is important to be clear that we have much more serious problems in the areas controlled by the Ansar Allah authorities. Last year, they issued more than 200 regulations on humanitarian action."
The Houthi "authorities asked NGOs to sign agreements that would not be consistent with humanitarian principles. About 40 percent of NGO projects last year went unapproved - that is 10 percent more than in the south. It has also been suggested that NGOs pay a 2 percent tax to fund the authorities' aid coordination body.
"The situation is unacceptable. Stopping the world's largest aid operation would be fatal for millions of people" in the war-torn country, the UN humanitarian chief warned.
"Peace is the only way to end the humanitarian crisis in Yemen... We are now entering the sixth year of this war. Enough is enough," Lowcock concluded.