When the jets from the Saudi-led coalition bombing Yemen swooped in, they hit the gas tanker with pinpoint accuracy.

It blew up, obliterating its contents, its driver, and everything around it.

Unfortunately for residents nearby, and the drivers of cars stopped behind it, that included them.

The men, women and, according to residents and Amnesty International, at least four children who died were among scores of civilians killed last week in one of the most disturbing and baffling of all the Middle East's conflicts.

The fight between the Sunni Arab world and the Shia Houthi rebels that have seized much of Yemen is baffling because it is not clear what either side hopes to gain.

It is disturbing because even as civilians are killed, the economy destroyed, and the war becomes unwinnable, a third party is rubbing its hands.

Al-Qaeda in the Arabian Peninsula (AQAP) is using the chaos to regain territory lost in recent months and mount new attacks.

"The Saudis will weaken the Houthis while we sit back and watch like a movie," an AQAP spokesman told The Telegraph. "Both sides are doing us a favour."

The attacks by Saudi Arabia and seven other Sunni Arab allies of the recognised Yemen government are now in their second week.

There is no sign though of a swift end to the crisis. Far from coming to terms, the Houthis, a militia that its enemies say is being used by Iran to create a new Persian empire on Saudi Arabia's doorstep, are marching on.

In the last week, they seized an army base overlooking the Bab al-Mandab – the sea lane that leads to the Red Sea and the Suez Canal – and attacked the centre of Aden. They briefly occupied the Crater, the zone named after the oceanic volcano's rim around which it is built that was once the heart of the British colonial administration.

There is no sign of its hold faltering on the two other major cities, Sana'a and Taiz.

The Houthis, too, are not squeamish about civilian casualties. They have fired on protesters and are also shelling civilian areas, witnesses said.

But the scale of casualties from air raids has shocked the world. The Saudis are using American and British planes – but without the attention to avoiding civilian deaths boasted of by the West in recent years.

The attack on Yareem targeted a number of fuel points. The day before, jets hit the Al-Mazrak refugee camp in the north, killing 40.

On Tuesday, it was the turn of a food factory complex in the town of Hodeida, including a dairy and an oil processing plant.

"Three rockets hit our factories," said the oil plant's owner, Abdel Wahab Thabet. "There are forty people dead so far, and there are many burnt bodies in the hospitals.

"If the world saw the pictures from our factory, they would not support this war.

"They know it's a factory complex. It employed around 5,500 employees.

No one expected they would be crazy enough to use their military power to kill innocent people."

Over 500 people have been killed in the past two week, the United Nations announced on Friday.

The attacks have divided a nation already split between a host of political competitors, including the Houthis, their ally the former President Ali Abdullah Saleh, the internationally recognised President Abd Rabbdu Mansour Hadi, who has fled, the Muslim Brotherhood-linked Islah party, and AQAP.

Some Aden residents, seeing the Houthis as a hostile, northern and Shia force, demanded the coalition "double down" and send in ground forces.

"The defence needs ground troops," Majid Shuaibi, a local resident and reporter said. "The Houthis are well trained and well set while the resistance is mostly normal people with light weapons."

However, further north, residents and analysts say the bombing is turning a population with little natural sympathy for the Houthis against their elected president.

"People are scared," said Sama'a al-Hamdani, a, analyst formerly based in Sana'a but currently in the United States. "The ground is shaking.

"This kind of warfare radicalises people. This is going to put people on the other side from the Saudis. Whatever process the Saudis undertake to form a government, the risk is that it's going to be looked at as a puppet government."

The Saudis say they had no choice but to intervene after the Houthis drove out Mr Hadi, whose 2012 election followed complex negotiations backed by the Gulf states and the international community.

Prince Saud al-Faisal, the Saudi foreign minister, last week said Yemen's collapse threatened the whole region.

"We are not warmongers, but if the drums of war call for it, we are prepared," he said. "The Houthi militias and the former president, with Iranian support, insist on messing in Yemen."

The problem is that neither side appears to have an end-game. Those close to the Saudi leadership say it is hoping to force the Houthis to the negotiating table – but at the same time refers to the Houthis as "gangsters", suggesting little desire to make concessions.

Meanwhile the Houthis evince no master plan for Yemen, including how to hold the vast majority that is hostile to them.

"Al-Qaeda are the only winners," Ms Hamdani said. "They are against the Saudi state, they are against the Yemeni state, they are against the Houthis – and they are looking like the people who had nothing to do with the war."

As if to prove the point, AQAP on Thursday seized the capital of Hadramaut province, Mukalla, freeing hundreds from its prison. One of those released was an AQAP emir, Khaled Batarfi, who was subsequently photographed holding cheerful court in the governor's palace.

When asked whether al-Qaeda was now operating with impunity, its spokesman was cautious. "I'm sure the Saudis and the Americans have a Plan B," he said.

That is, if there is even a Plan A.

Telegraph