"The Yemeni people's suffering has relentlessly intensified", he told the UN Security Council yesterday.
The Saudi-led coalition had been named on the blacklist previous year after the United Nations report blamed it for 60 percent of child deaths and injuries in Yemen in 2015 and half the attacks on schools and hospital.
Since 2015, Yemen has been the site of a devastating war between the Saudi-backed government and Huthi rebels allied with Iran. The Houthi movement denies the allegations. O'Brien said monthly air strikes in 2017 had tripled from previous year, while monthly reports of armed clashes were up 50 percent.
"Death looms for Yemenis by air, land and sea", said the United Nations special envoy to Yemen, Ismail Ould Cheikh Ahmed. The United States regards al Qaeda in Yemen as one of the deadliest branches of the militant network.
"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. Since then, the Iranian-backed Houthis have been dislodged from most of the south, but remain in control of Sanaa and much of the north.
Before the conflict, Yemen had been making progress, with fewer people hungry and rising school enrolment, O'Brien said in his statement. The World Health Organization said Monday that the cholera outbreak has killed 2,000 people and infected an estimated 500,000.
In his recent meetings with senior Yemeni officials and regional leaders, he said that "there is still consensus on the need to reach a political solution and to support the UN-sponsored peace process under the Secretary-General's auspices". Despite the successful interception, the launching of this missile, which made it 43 miles south of Mecca, illustrates the extent to which Saudi Arabia and its regional allies have failed to achieve a key objective in their war in Yemen: to deprive the Houthi militants of the means to threaten Saudi national security with heavy weaponry and missiles sourced domestically and overseas.