Warplanes from the Saudi-led coalition fighting Iran-backed rebels in Yemen hit positions of pro-government forces by mistake on Monday despite a humanitarian truce, killing a dozen people, military sources said.

Clashes also broke out on the ground in several areas of the impoverished country in spite of UN calls for all sides to respect a ceasefire to allow desperately needed aid deliveries.

The Shiite Huthi rebels, who control swathes of Yemen including the capital Sanaa, said they had not been consulted about the truce that began at midnight (Sunday 2100 GMT).

The Arab regional coalition, which has waged four months of air strikes in support of exiled President Abedrabbo Mansour Hadi, announced the five-day ceasefire to allow emergency supplies to flow into the war-scarred nation.

It reserved the right to respond to "military activity or movement".

Although there were no reports of air strikes on the rebels on Monday, military sources reported a "friendly fire" incident in which coalition warplanes hit positions of Hadi loyalists in the southern province of Lahj, killing 12 people.

At least 30 others were wounded in the strikes on hills overlooking the rebel-held Al-Anad airbase, as well as in nearby Radfan, the sources said.

There was no immediate comment by the Saudi-led coalition.

EU foreign affairs chief Federica Mogherini held talks Monday in Riyadh with Saudi Foreign Minister Adel al-Jubeir, and both said they agreed on the need for a political solution in Yemen.

Overnight the Huthis bombarded areas in the southern provinces of Taez, Lahj and Dhaleh, according to witnesses and military sources.

Rebel tanks fired on residential areas in Jebel Sabr in Taez, sparking clashes with loyalist troops, witnesses said. Military sources said 11 rebels, five civilians and four loyalists were killed.

Loyalists were reported to have seized territory on the northern outskirts of Aden, after expelling the rebels from the southern port city last week following four months of ferocious fighting.

The pro-government forces have been bolstered by new weaponry and armoured vehicles delivered by the coalition.

- Truce 'not long enough' -

UN chief Ban Ki-moon on Sunday made a plea for all sides to "agree to and maintain the humanitarian pause for the sake of all the Yemeni people".

But Mohammed Ali al-Huthi, the self-described "president of the High Committee of the Revolution", a body formed by Huthi militants, said his group had not been consulted by the UN about the ceasefire.

The group could therefore not give a "negative or positive" answer about the truce, he said.

The Red Cross called on all parties to respect the ceasefire, and urged a longer truce.

"We would like to see a longer and stable truce, which is respected from all sides to allow humanitarian agencies to reach all affected areas," said the Red Cross spokesman in Sanaa, Adnan Hizam.

"Five days are not enough to cover the humanitarian needs," he told AFP.

The United Nations says the conflict has killed more than 3,640 people, around half of them civilians, since late March.

Johannes Van Der Klaauw, the UN humanitarian coordinator for Yemen, made an urgent appeal for humanitarian access, saying after a visit to Aden that the city was "devastated".

"I repeat my plea to all parties of the conflict to put an end to the attacks on civilians and to end the destruction of critical infrastructure, vital for supplying essential goods and services to the civilian population," he said in a statement.

"The UN and its partners are committed to rapidly scaling up the humanitarian response effort across the whole of Yemen," he added.

A UN-declared six-day truce failed to take hold earlier this month after it was ignored by both the coalition and the rebels.

Relief supplies, however, have begun to trickle into Aden after loyalist fighters secured the city, which had been Hadi's last refuge before he fled to Saudi Arabia in March.

Several ships have docked in Aden since last Tuesday carrying thousands of tonnes of aid supplies sent by the UN World Food Programme and Gulf nations.

But distributing the aid, particularly outside the city, poses a major challenge.