UN chief Ban Ki-moon on Monday called for an immediate humanitarian truce in Yemen as peace talks began in Geneva and a Saudi-led Arab coalition continued aerial bombardments of the ravaged country.

Global powers are keen for a speedy resolution of the conflict, fearing the growing power of Al-Qaeda in the Arabian Peninsula, the Yemeni branch of the jihadist network which has taken advantage of the chaos to seize territory.

"We do not have a moment to lose," Ban said, adding that the fighting was "giving strength to some of the world's most ruthless terrorist groups."

"I hope this week starts the beginning of the end of the fighting," Ban said, stressing this was vital to get crucial humanitarian supplies through.

The UN has described the country's humanitarian crisis as "catastrophic", with 80 percent of the population -- 20 million people -- in need of aid.

"Ramadan begins in two days," Ban said, stressing that the holy Muslim month should be a period for harmony, peace and reconciliation.

"I have emphasised the importance of another humanitarian pause for at least two weeks," the UN chief said, after meeting the government delegation.

Yemen has been wracked by conflict between Iran-backed Shiite rebels and exiled President Abedrabbo Mansour Hadi's internationally recognised government.

Ban said the violence had killed more than 2,600 people, half of them civilians.

- 'Yemen a time bomb' -

"Today, Yemen's very existence hangs in the balance," Ban said. "While parties bicker, Yemen burns."

He also called on the warring factions to "reach agreement on local ceasefires, with the withdrawal of armed groups from cities."

But analysts said there was little optimism over the talks, which began on Monday without the presence of the Iran-backed rebel delegation after their plane was delayed in Djibouti.

The UN announced later on Monday that the UN plane carrying the Huthi representatives was finally on its way to Switzerland after the layover in Djibouti.

The UN had worked to get the rebel group to Geneva on time but "there are issues involving international aviation that are beyond the control of the UN", spokesman Stephane Dujarric said.

Pre-dawn air strikes meanwhile targeted Huthi rebel positions in the main southern port city of Aden and continued throughout Monday, residents said, as fighting raged on between rebel forces and pro-government fighters.

Rebels fired Katyusha rockets at a residential part of Aden's Mansura district, said Ali al-Ahmedi, spokesman of the pro-government Popular Resistance -- a coalition of pro-government fighters, Sunni tribes and southern separatists.

Medics at a nearby hospital said that three members of the same family were killed and four others were wounded in the attack.

AFP could not immediately verify the toll given by Popular Resistance sources of 20 rebels and seven pro-government fighters killed in Aden.

Underscoring the need for immediate action in Yemen, Ban said: "The ticking clock is not a time piece, it is a time bomb."

- Breakthrough unlikely -

The rebels, supported by military units loyal to ex-president Ali Abdullah Saleh, have seized control of large parts of the country including the capital Sanaa, forcing Hadi to flee to Saudi Arabia in February.

Yemen Foreign Minister Riad Yassin said the prospects were for a breakthrough were poor.

"I'm not very optimistic," Yassin told AFP, adding that they (the rebels) "never respect any treaty".

Yassin likened the rebels to the Islamic State group or Nigeria's Boko Haram and accused them of sending far more representatives and advisors to the Geneva talks than had been agreed.

"They want to come here to make chaos," he said.

The government delegation led by the foreign minister reiterated its demands on Sunday night, including that any ceasefire must see the militias withdraw from all territory they have conquered.

Fearing an Iran-friendly regime on its southern border, Riyadh has been leading a campaign of air strikes against the rebels since March 26 but has so far failed to rout them from territory they have seized.

Any chance of success at the first talks between the warring factions since Hadi fled Sanaa was "very unlikely", Yemen expert Laurent Bonnefoy said, adding: "Indeed, each side appears to be inflexible, disinclined toward compromise".