Kuwait City - Crucial talks between Yemen's government and rebels were delayed on Monday after the insurgents failed to show up, prompting UN concern over the fate of peace efforts for the war-torn country.
The talks in Kuwait aim to bring an end to more than 13 months of fighting that have devastated already-impoverished Yemen.
A week-old ceasefire was meant to lay the groundwork for the talks, but it has been repeatedly breached, with both sides trading blame.
Representatives of the internationally recognised government as well as the Iran-backed Huthi rebels and their allies - loyalists of former president Ali Abdullah Saleh - were meant to gather on Monday morning in Kuwait for the negotiations.
But while a government delegation led by Yemeni Foreign Minister Abdulmalek al-Mikhlafi was in Kuwait, the rebel delegation remained in Yemen.
"Due to developments over the last few hours, the start of the Yemeni peace negotiations scheduled to begin today... will be delayed," UN envoy Ismail Ould Cheikh Ahmed said in a statement, without specifying when they might take place.
"The next few hours are crucial," he said, calling on all parties to "take their responsibilities seriously and agree on comprehensive solutions".
A source close to the government delegation in Kuwait City told AFP that the rebels "haven't left Sana'a and are procrastinating."
Speaking from Yemen's rebel-held capital, a source from the Huthi political bureau confirmed to AFP that the rebels, expected in Kuwait alongside representatives from Saleh's General People's Congress party, had not left Sana'a.
"They did not go to the Kuwait talks because of the continued Saudi aggression on Yemen," said the source, who requested anonymity. "Saudi Arabia did not commit to the truce."
Saudi Arabia is leading a military coalition of Arab Sunni states which has been supporting pro-government forces with air strikes, weapons and troops since March last year.
It launched the intervention after the rebels, a Shiite minority group that has long complained of marginalisation, descended from their northern stronghold to seize control of Sana'a in 2014.
As they advanced into other areas, President Abedrabbo Mansour Hadi and other officials fled first to the main southern city of Aden and eventually to Riyadh.
The loyalists have since managed to reclaim large parts of the south, establishing a temporary capital in Aden, but have failed to dislodge the Huthis from Sana'a and other key areas.
Prior to the UN announcement of a delay, the Saudi government voiced hope for "the success of consultations" in Kuwait, in a statement published on the official Saudi Press Agency.
The UN's Ould Cheikh Ahmed urged the Huthi rebels and their allies not to "miss this opportunity that could save Yemen the loss of more lives".
He had earlier expressed hopes for the talks, telling the UN Security Council on Friday that Yemen has "never been so close to peace".
The situation across Yemen was relatively calm on Monday despite skirmishes in several areas, local sources said.
Previous attempts at peace talks - including a failed round in January - have been unable to stop the conflict, which the United Nations says has killed more than 6 400 people and forced almost 2.8 million from their homes.
Among the issues meant to be tackled in Kuwait are security arrangements, the withdrawal of militias and armed groups, the handover of heavy weapons and the release of detainees.
The conflict in the impoverished nation has raised tensions between Sunni Arab states and Shi'ite powerhouse Iran.
Jihadists including from Al-Qaeda in the Arabian Peninsula (AQAP), the powerful Yemeni branch of the extremist network, have also exploited the conflict to seize territory and gain influence.