SANAA: The Saudi-led coalition launched new airstrikes in at least two Yemeni provinces on Monday, piercing a humanitarian pause that started at midnight Sunday, security officials and witnesses said.

Security officials and field commanders of forces allied with the Saudi-led coalition said 15 of their fighters were killed in two separate coalition airstrikes in the province of Lahj.

The accidental strikes against allied forces wounded more than 40 fighters, they say, adding that the death toll is expected to rise.

The deadly strikes happened near the strategic military base of al-Anad, which is held by the rebels and was also hit by coalition planes Monday. The coalition also struck north of the port city of Aden.

Airstrikes had been halted for hours but ground fighting erupted in multiple provinces within minutes of the start of the unilateral cease-fire late Sunday.

The rebels, said in a statement that they fired missiles across the border at a Saudi military position in the kingdom's Jazan region. The Saudi-owned Al-Hadath news channel said Saudi forces “responded” to Houthi shelling in Jazan, without elaborating.

Fierce clashes also broke out in the nearby town of Sabr, which is on a key supply route. Officials aligned with pro-government fighters say they have regained control of the center of the town and were trying to advance into northern neighborhoods. Witnesses who fled Sabr on Monday morning say corpses of fighters lay in the street next to destroyed military vehicles.

Officials and witnesses said there were sporadic clashes in Yemen's central Marib province. In Taiz, mortar shells fired in the center of the city killed four civilians, security and medical officials said.

Anti-aircraft gunfire was heard in the rebel-held capital, Sanaa, as coalition planes buzzed overhead.

The humanitarian pause was intended to help allow humanitarian aid to be delivered to ease the suffering of civilians in the Arab world's poorest country.

All officials spoke on condition of anonymity because they were not authorized to speak to reporters.

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.

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.