"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 past year.
Yemen also faces a deadly cholera outbreak and stands at the brink of starvation.
The number of air strikes on Yemen in the first half of 2017 has already exceeded the total for 12 months previous year, according to a United Nations report released Monday.
O'Brien said 17 million Yemenis don't know where their next meal is coming from, nearly 7 million are facing the threat of starvation and almost 16 million lack access to clean water and sanitation.
"I renew my call.to address the following points: ensure that all ports - land, sea and air - are open to civilian - including commercial - traffic", O'Brien said, adding that the airport in the capital Sanaa should be opened "immediately" to humanitarian aid.
More than half a million Yemenis have been infected with cholera and some seven million are on the brink of starvation, the United Nations has said.
DPRK slams South Korean president for north policy
Steve Bannon, the White House's chief strategist, told The American Prospect that there was no military solution to Pyongyang's nuclear threat.
Despite worldwide support for a peaceful settlement, certain parties in the conflict continued to "place personal interests ahead of the urgent need for peace", said Cheikh Ahmed, who briefed the Security Council via video link from Amman, Jordan.
But he said the United Nations will continue working with Yemenis "to mainstream the language of peace and reach a political solution".
"This human tragedy is deliberate and wanton - it is political and, with will and with courage which are both in short supply, it is stoppable", he said, reiterating the UN's ongoing calls for a political solution to the conflict.
On July 27, the Saudi-led coalition's air defense forces shot down a ballistic missile in Taif province, which Houthi rebels fired from Northern Yemen. This not only includes the Houthis concentrated in the north but also the southerners who live in the territory the People's Democratic Republic of Yemen (aka South Yemen) governed from 1967-1990.
The Yemen war has also taken a heavy toll on the country's facilities and infrastructure, destroying many hospitals, schools, and factories.