"It is simply wrong to insist these cargoes go to Aden, not Hodeidah", O'Brien told the U.N. Security Council, appealing for countries to back a U.N. mechanism that started inspecting commercial shipments to rebel-held ports in May a year ago.
More than 8,300 people have been killed and 44,000 wounded since the Saudi-led coalition entered the Yemen war in 2015.
The Saudi-led coalition began an air campaign in Yemen in March 2015 to defeat Iran-allied Houthi rebels. The Houthi movement denies the allegations.
"Death looms for Yemenis by air, land and sea", said the United Nations special envoy to Yemen, Ismail Ould Cheikh Ahmed.
Yemen was hit more more airstrikes in the first half of this year than in the whole of 2016, increasing the number of civilian deaths and forcing more people to flee their homes, global humanitarian agencies reported.
Al Qaeda has exploited the conflict to try to deepen its influence in Yemen, repeatedly launching bomb and gun attacks.
Stephen O'Brien urged the immediate reopening of the airport in the capital Sanaa to civilians and the lifting of restrictions on commercial imports coming through the port of Hodeida, which handles some 70 per cent of goods coming into Yemen. Sanaa and Hodeidah best serve the north, not Aden.
O'Brien said that he was grieved that, in the past two years, despite his and his team's much effort, he has been "unable to report any significant improvement in the deplorable, avoidable and completely man-made catastrophe that is ravaging Yemen".
The World Health Organisation (WHO) has said that the number of suspected cholera cases in Yemen has reached half a million, while the death toll has exceeded 2,000.
Ould Cheikh Ahmed told the Security Council on Friday he is working to get a deal to secure the flow of aid, payment of salaries to civil servants and prevention of arms smuggling.
But he said the United Nations will continue working with Yemenis "to mainstream the language of peace and reach a political solution". Each of the United States and Saudi Arabia have accused Houthi militias of receiving Iranian funds and arms.