By Emma Batha

LONDON (Thomson Reuters Foundation) - The number of children killed in Yemen's conflict has risen sharply in the last week, a U.N. official said on Monday as aid agencies condemned a weekend airstrike on a school which killed 10 children and injured 28.

A Saudi-led coalition, which began a military campaign in March last year against Iranian-allied Houthi rebels that drove the internationally recognized government into exile, said the strike in Saada province had targeted a Houthi training facility.

But UNICEF, which visited the site in Haydan after the strike as well as the hospital treating the injured children, dismissed suggestions the victims were Houthi recruits.

"The children who were killed were between the ages of six and 14, and the majority were between six and eight," said Julien Harneis, UNICEF's Yemen representative.

"The Houthis do not recruit children so young into their militia. We spoke with the parents, we checked the ages and we visited the site, and there is nothing to indicate it was anything other than a Koranic school."

Harneis welcomed calls by the U.N. Secretary General Ban Ki-moon on Monday for a swift investigation into the attack. [L8N1AW1BD]

"The main thing is there should be change in the way the conflict is conducted. How did this happen? Why did it happen? What needs to change so that more children will not be killed?"

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

Aid agencies say well over 2,000 children have been killed or wounded since the start of the war.

A recent U.N. report said the coalition was responsible for 60 percent of child deaths and injuries last year.

Harneis said the number of children killed and injured by airstrikes, street fighting and landmines had increased sharply during "a massive spike in violence" across Yemen in the 10 days since peace talks ended in Kuwait without a breakthrough.

PRESSURE COOKER

He said the humanitarian situation was deteriorating as violence escalated and the country became increasingly cut off.

"You have got this real pressure cooker with a collapsing economy and a collapsing health system," Harneis said.

"All across the board, it's just getting worse. I have never seen anything so bad. It's just appalling."

He said the central bank was running out of foreign exchange and Yemeni rials were difficult to obtain, jeopardizing aid operations.

Harneis said the impact of the war on healthcare would mean an additional 10,000 children under five would die this year from preventable causes such as diarrhea and measles.

He also warned that the immediate crisis would have grave long term implications.

"If the health system and education systems collapse then for years after the war ends, the country will be vulnerable to things like pandemics, and you will have lower education rates which is going to hamper the country for years, if not decades."

Grant Pritchard, advocacy director at Save the Children's Yemen office, said the attack on the school was "inexcusable".

He said well over 100 schools and hospitals had been damaged and destroyed with nearly half the attacks blamed on coalition strikes.

"It's difficult to understand how they could mistake a military training camp (for) a school," he added.

(Editing by Katie Nguyen.; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, which covers humanitarian news, women's rights, trafficking, corruption and climate change. Visit news.trust.org to see more stories.)