The UN envoy to Yemen, Martin Griffiths, offered a bleak picture of prospects for peace in the country, on Tuesday.
This came at the end of his three-year stint in a peacemaking role in which little progress was made towards ending the war.
In his final address to the UN Security Council as envoy to Yemen, Mr Giffiths said he had been unable to bridge differences between Yemen's internationally recognised government, which is backed by a Saudi-led coalition, and the Iran-backed Houthi rebels.
Mr Griffiths is set to become the head of UN humanitarian operations in the coming weeks, after his latest bout of shuttle diplomacy failed to end a Houthi assault on the oil and gas-rich province of Marib, the government’s last stronghold in northern Yemen.
“Yemen is a tale of missed and then lost opportunities,” Mr Griffiths said.
“Over the course of the conflict, armed and political actors have multiplied and fragmented.
"Foreign interference has grown. What was possible in terms of conflict resolution years ago is not possible today. And what is possible today may not be possible in the future.”
Mr Griffiths said his most recent talks with Houthi officials in the rebel-held capital Sanaa did not alter their long-standing demand for the lifting of an air and sea blockade before they would agree to ceasefire negotiations.
“Perhaps an international conversation may need to restate the realistic goals for a negotiation process,” he told the 15-member council.
Diplomats met in New York amid ongoing Houthi violence, including a drone attack on a school in south-western Saudi Arabia and drone and missile strikes that killed at least 10 civilians in Marib on Thursday.
"I strongly condemn these acts, as well as the Houthis' continued crackdown on the rights and freedoms of Yemeni women, as well as religious and ethnic minorities," said Barbara Woodward, the UK's ambassador to the UN.
“I think it's clear that the Houthis do not have the courage to embrace a ceasefire.”
The US State Department on Tuesday said its Special Envoy for Yemen Tim Lenderking will travel to Saudi Arabia this week to meet Yemeni and Saudi senior officials, as well as Mr Griffiths.
Mr Lenderking "will discuss the latest efforts to achieve a comprehensive, nationwide ceasefire, which is the only way to bring Yemenis the relief they so urgently need," the State Department said.
Yemen's war erupted in late 2014 when the Houthis seized large parts of the country, claiming to be fighting corruption and foreign interference, while imposing Iran-style restrictions on areas under their control.
Fighting intensified in March 2015 when a Saudi-led coalition launched an air strike campaign to try to restore the ousted government of President Abdrabu Mansur Hadi.