By Gabriel Gatehouse BBC Newsnight

The BBC's Gabriel Gatehouse has just returned from Yemen, where he had rare access to the scale of the humanitarian crisis caused by what some are calling the "forgotten war".

The bombing campaign in the skies over Yemen is in its sixth month now. Every day, residents of the capital, Sanaa, listen nervously for the rumble of jets. They know what will follow: a flash in the sky, a sickening pause, then the thud and boom of explosions as the missiles strike.

The aim of the Saudi-led coalition is to oust Houthi rebels from the city. The Houthis took Sanaa last year, with the help of forces loyal to Yemen's ousted former president Ali Abdullah Saleh and the support of Iran.

In response, Saudi Arabia, Iran's regional rival, has imposed a blockade on the north of the country, controlling what comes in by land, sea and air.

"Yemen is one of the world's worst crises," says Tariq Riebl, head of programmes in Yemen for the charity Oxfam.

"We have bombings every single day via airstrikes. We have ground fighting of very heavy levels. The country is facing famine which could start in a couple of weeks or months if things continue the way they are. We have more than a million people displaced across the country."

Away from the capital, towards the Saudi border, the bombing is even more intense. We travelled across the mountains that ring Sanaa to investigate reports of deliberate attacks on civilians - attacks that could amount to war crimes.

On the evening of 29 August, Ahmed Al Beyna, 13, had dinner as usual with his parents and his brother Mohammed. They live in the village of Alrabu Matwara, near the town of Abs in the north-west of the country. Just before 6pm, the brothers set off for work at a nearby water bottling plant.

They were coming to the end of their night shift when the missile struck.

"There was an explosion, then everything burst into flames," says factory worker Khalid al-Hababi.

"Most of the workers came out like pieces of coal, buried beneath the rubble."

"Everything is gone," said Akram, another worker who witnessed the aftermath. "There's nothing left. No factory, no people. We found the workers burned on to the machines."

When we arrived, two days after the strike, smoke was still rising from piles of molten plastic.

The owner showed us a list of who was working that night. More than half were killed, 13 men in total, including Ahmed and his brother.

The Saudis said the bottling plant was in fact a weapons factory and a training camp for African mercenaries. We saw no evidence of that.

The bottling plant near Abs is just one of dozens of civilian targets that have been hit since the Saudi-led campaign began at the end of March.

The British dimension

The airstrikes are backed by a resolution at the United Nations Security Council. But the UN's top humanitarian official in Yemen, Johannes van der Klaauw, says attacks on civilian infrastructure are violations of the laws of war.

"Schools and hospitals, markets, enterprises and factories should not be stricken, should not be shelled. Even in warfare there are certain rules, and they are being violated in this conflict," he said.

Since the conflict started, more than 2,000 civilians have been killed. Some are victims of months of vicious ground fighting between the two sides. Houthi soldiers, some of them no more than teenagers, are accused of firing heavy weapons in built-up areas.

But it is the Saudis and their coalition partners, mainly Gulf Arab countries including the United Arab Emirates, who have overwhelming force.

The coalition's efforts are supported by Britain and the United States. Both countries continue to sell arms to Saudi Arabia, and are providing the Saudi-led coalition with liaison officers and technical support.

Oxfam, whose own warehouse in Yemen has been hit by an airstrike, says the UK could be in breach of domestic and international laws on the sales of arms.

"It's difficult to argue that a weapon sold to Saudi Arabia would not in some way be used in Yemen," says Mr Riebl.

"Or if it's not used in Yemen it enables the country to use other weapons in Yemen."

The Arms Trade Treaty, which came into force in December 2014, prohibits the sale of weapons where there is a clear risk they could be used for war crimes.

The regional context

The British government says it is not participating directly in the Saudi-led operations, but acknowledges it is providing technical support and precision-guided weapons to Saudi Arabia. The Saudi Air Force has a fleet of British and American-made fighter jets, including F15s, Tornados and Eurofighter Typhoons.

In July, a consignment of Paveway IV missiles, a highly accurate 500-pound bomb originally earmarked for the RAF, was delivered to Saudi Arabia.

"The UK is digging into its own weapons supplies to replenish Saudi stocks," says Michael Stephens of the Royal United Services Institute (RUSI).

The majority of the Saudi airstrikes are being carried out by American-made F15 jets. The Saudi-led coalition has publicly given few details of which weapons are being used in Yemen. Saudi Arabia is also involved in the air campaign over Syria in airstrikes against Islamic State targets. But Mr Stephens says British missiles are being used in Yemen.

"They're firing UK-supplied weapons," he says.

The combined effects of intense ground fighting, airstrikes and the blockade are having a devastating effect on the country.

A million and a half people have fled their homes, seeking shelter in makeshift camps. Half the population of Yemen doesn't know where the next meal will come from.

That number now includes Ibrahim and Khadija Al-Beyna, the parents of Ahmed and Mohammed, who were killed in the airstrike on the water bottling plant. The brothers were the only breadwinners in a family of nine.

Ahmed was a typical teenager, his mother, Khalidja, remembers: always on his bike or chasing pigeons.

"They took our children," says Ibrahim. "Not one but two. Mohammed and Ahmed. Together, in one moment, in one day. Whoever did this to us, may God repay them."

In recent days, the airstrikes have intensified. In the capital, there is talk of an impending advance by pro-Saudi forces.

This war is part of the wider regional struggle for power between Saudi Arabia and Iran.

In Sanaa, the Houthis do not enjoy universal support - far from it. But with every airstrike, and every civilian death, resistance to the Saudis and their allies is growing.

Part two of Gabriel Gatehouse's special report from Yemen is on Newsnight on Friday 11 September. You can also catch up on on iPlayer afterwards.