UN-sponsored talks aimed at ending the conflict in Yemen by bringing representatives of the warring factions to Geneva are expected to begin on Monday, the world body said.

"The United Nations takes this opportunity to appeal to Yemen's political actors to participate in these consultations in good faith and without pre-conditions, and in a climate of trust and mutual respect," said the UN's peace envoy for Yemen, Ismail Ould Cheikh Ahmed.

Announcing the start of "preliminary inclusive consultations" in Geneva on Monday, the envoy said in a statement the talks would bring together representatives of the exiled government, the rebel Iran-backed Huthis, former president Ali Abdullah Saleh's General Peoples' Congress and other opposition groups.

The internationally-recognised government of President Abd-Rabbo Mansour Hadi said its delegation flew to Geneva on Saturday, but representatives of the Shiite Huthis and Saleh refused to board a UN plane from Sanaa to Geneva on Saturday.

A Huthi official said the rebels' refusal was grounded in the fact the plane was due to stop off in Saudi Arabia -- which is leading a fierce campaign of air strikes against them.

UN spokesman Ahmad Fawzi told reporters that representatives of the two sides were expected to be in the Swiss city later Sunday.

The delegations would "start with what we call proximity talks in two separate rooms with the hope they can be brought together", Fawzi told reporters, adding that there would be "extremely intensive consultations day and night".

In Geneva on Sunday for preliminary meetings, UN Secretary General Ban Ki-moon is due to take part in the opening session of the talks on Monday before returning to New York.

"We still hope that the parties will observe a humanitarian pause," Fawzi said.

The UN Security Council and Ban have both called for a renewed humanitarian halt in the fighting following May's truce.

The talks are aimed at securing a ceasefire, agreeing on a withdrawal plan for the Huthis and stepping up deliveries of humanitarian aid.

The Security Council this week heard a report from new UN aid chief Stephen O'Brien, who described Yemen's humanitarian crisis as "catastrophic," with 20 million civilians in need of aid -- 80 percent of the population.