SANAA/RIYADH/Aden - Saudi-led air strikes on a school in a rebel-held province of northern Yemen have killed 10 children and wounded 28 others, Doctors Without Borders (MSF) said on Sunday.

"We received 10 dead children and 28 wounded, all under the age of 15, who are victims of air strikes on a Koranic school in Haydan," in Saada province, said MSF spokeswoman Malak Shaher, adding the attack took place on Saturday.

Shaher told AFP that MSF had received the children at a field hospital near the school before they were transferred to a public hospital. The Iran-backed Huthi rebels posted pictures and videos on Facebook of dead and bloodied children wrapped in blankets.

Huthi spokesman Mohammed Abdulsalam said warplanes "targeted" children at the Jomaa bin Fadhel school, in what he described as a "heinous crime".

The United Nation's children agency, UNICEF, confirmed the attack warning that "with the intensification in violence across the country in the past week, the number of children killed and injured by air strikes, street fighting and landmines has grown sharply."

"UNICEF calls on all parties to the conflict in Yemen to respect and abide by their obligations under international law," it said.

"This includes the obligation to only target combatants and limit harm to civilians and civilian infrastructure."

Saudi Arabia reacted angrily to a decision in June to blacklist the coalition after a UN report found the Arab alliance responsible for 60 percent of the 785 deaths of children in Yemen last year.

UN chief Ban Ki-moon had accused Saudi Arabia of threatening to cut off funding to UN aid programmes over the blacklist, a charge denied by Riyadh.

The UN says more than 6,400 people, mostly civilians, have been killed in Yemen since the coalition air campaign began in March last year.

The Saudi-led coalition on Sunday denied targeting a Yemeni school in air strikes that killed 10 children, instead saying it bombed a camp at which Iran-backed rebels train underage soldiers.

Doctors Without Borders, a Paris-based relief agency also known as MSF, said the children were killed Saturday in coalition air raids on a school in Haydan, a town in rebel-held Saada province.

The coalition of Arab states has been battling the Huthi rebels since 2015 when the insurgents seized Sanaa before expanding to other parts of the country.

Ten days ago it acknowledged "shortcomings" in two out of eight cases it has investigated of strikes on civilian targets in Yemen that the UN has condemned.

Coalition spokesman General Ahmed Assiri said the strikes hit a Huthi training camp, killing militia fighters including a leader identified as Yehya Munassar Abu Rabua. "The site that was bombed... is a major training camp for militia," he told AFP. "Why would children be at a training camp?"

Yemen's government had confirmed to the coalition that "there is no school in this area," he said.

Assiri said MSF's toll "confirms the Huthis' practice of recruiting and subjecting children to terror."

"They... use them as scouts, guards, messengers and fighters," Assiri said, noting previous reports from Human Rights Watch on the rebels' use of underage recruits.

"When jets target training camps, they cannot distinguish between ages," he added. MSF spokeswoman Malak Shaher said those killed in the strikes on "a Koranic school" were all under the age of 15.

In the other side, Yemeni army forces backed by Arab coalition aircraft killed about 40 suspected al Qaeda fighters on Sunday as they fought their way into two militant strongholds in eastern Yemen, a local official and residents said.

Al Qaeda in the Arabian Peninsula (AQAP) has exploited a 16-month-old civil war between the internationally recognised government of President Abd-Rabbu Mansour Hadi and the Iran-allied Houthis to capture a 600-km (370-mile) stretch of Arabian Sea coastline in eastern Yemen.

Hadi's troops and forces from the Saudi-led Arab coalition drove out AQAP - widely considered the most dangerous branch of the global militant group - from the Hadramout provincial capital of Mukalla in April.  The militants have since repeatedly withdrawn from and then returned to Zinjibar and Jaar, the capital of Abyan and the province's second largest city.

Abyan Governor Al-Khader Mohammed al-Saidi, speaking by telephone from Jaar, told Reuters that three brigades took part in the operation and that troops have "taken complete control of both cities".

Saidi said that 40 AQAP members were killed in both cities, while the rest fled. He said three soldiers were killed and several were wounded in the operation.

"We met with citizens and fighters and both were happy to be free under government authority," the governor said.

Residents said an AQAP suicide bomber blew himself up in a car trying to attack troops in Zinjibar, a city of some 100,000 people, but no one else was hurt.

Saudi Arabia and its Gulf Arab allies intervened in the civil war in Yemen in March last year after the Houthis advanced on his headquarters in the southern port city of Aden and forced him to flee to Riyadh.

The war has killed more than 6,500 people, displaced more than 2.5 million and caused a humanitarian catastrophe in one of the world's poorest countries.

Coalition bombing had mostly focused on the Houthis and troops loyal to former President Ali Abdullah Saleh, but began turning their attention to AQAP earlier this year when forces funded and trained by the United Arab Emirates launched a surprise attack to win Mukalla.

But an armed push toward Qaeda-held towns in Abyan and neighbouring Lahj province proved more difficult, and militants launched repeated suicide attacks against Yemeni forces.

"We would have hoped MSF would take measures to stop the recruitment of children to fight in wars instead of crying over them in the media," he said.

The United Nation's children agency, UNICEF, also reported the attack.

It warned that "with the intensification in violence across the country in the past week, the number of children killed and injured by air strikes, street fighting and landmines has grown sharply."

The rebels posted pictures and videos on Facebook of dead and bloodied children wrapped in blankets. Assiri sent to AFP pictures of Huthi children carrying rifles and rocket-propelled grenade launchers.

Huthi spokesman Mohammed Abdulsalam said warplanes "targeted" children at the Jomaa bin Fadhel school, in what he described as a "heinous crime".

The Arab coalition launched its air war against the Huthis on March 26, 2015.

After a three-month pause, it resumed them on Tuesday, less than 72 hours after UN envoy Ismail Ould Cheikh Ahmed announced the collapse of peace talks.

Raids struck a food factory in Yemen's rebel-controlled capital, killing 14 people, according to medics.

The factory is near a military equipment maintenance centre targeted by the coalition.

The UN had also voiced concern over the increased fighting in the past week, warning that more than 80 percent of the population needs aid.

"UNICEF calls on all parties to the conflict in Yemen to respect and abide by their obligations under international law," it said.

"This includes the obligation to only target combatants and limit harm to civilians and civilian infrastructure."

Saudi Arabia reacted angrily to a decision in June to blacklist the coalition after a UN report found the alliance responsible for 60 percent of the 785 deaths of children in Yemen last year.

UN chief Ban Ki-moon had accused Saudi Arabia of threatening to cut off funding to United Nations aid programmes over the blacklist, a charge denied by Riyadh.

Coalition states have formed a 14-member investigative team which has probed claims of attacks on a residential area, hospitals, markets, a wedding and World Food Programme aid trucks.

It found the coalition guilty of "mistakenly" hitting a residential compound after receiving "imprecise" intelligence information and offered compensation to families of the victims.

The team also held the coalition responsible for air strikes on an MSF-run hospital, also in Haydan, but accused the rebels of having used the hospital as a hideout.

The UN says more than 6,400 people, mostly civilians, have been killed in Yemen since the coalition air campaign began in March last year.

The coalition meanwhile announced that Saudi air defences on Saturday intercepted a Scud missile fired from Yemen towards the kingdom.

Around 100 members of the Saudi forces and civilians have been killed inside the kingdom's borders since the coalition campaign began.