The embargo requires foreign nations to take measures to ensure they do not militarily assist anyone in the conflict in Darfur, in which the UN estimates that about 300,000 people have died.

More than two million people are also believed to have fled their villages in Darfur, destroyed by pro-government Arab Janjaweed militia.

Panorama traced the first lorry by travelling deep into the remote deserts of West Darfur.

They found a Chinese Dong Feng army lorry in the hands of one of Darfur's rebel groups.

The BBC established through independent eyewitness testimony that the rebels had captured it from Sudanese government forces in December.

The rebels filmed a second lorry with the BBC's camera. Both vehicles had been carrying anti-aircraft guns, one a Chinese gun.

Markings showed that they were from a batch of 212 Dong Feng army lorries that the UN had traced as having arrived in Sudan after the arms embargo was put in place.

The lorries came straight from the factory in China to Sudan and were consigned to Sudan's defence ministry. The guns were mounted after the lorries were imported from China.

The UN started looking for these lorries in Darfur three years ago, suspecting they had been sent there, but never found them.

"We had no specific access to Sudanese government army stores, we were not allowed to take down factory codes or model numbers or registrations etc to verify these kinds of things," said EJ Hogendoorn, a member of the UN panel of experts that was involved in trying to locate the lorries.

Culpability

China has chosen not to respond to the BBC's findings. Its public position is that it abides by all UN arms embargoes.

China has said in the past that it told Sudan's government not to use Chinese military equipment in Darfur.

Sudan's government, however, has told the UN that it will send military equipment wherever it likes within its sovereign territory.

An international lawyer, Clare da Silva, says China's point that it has taken measures in line with the arms embargo's requirements to stop its weapons from going to Darfur is meaningless.

"It is an empty measure to take the assurances from a partner who clearly has no intention of abiding by the resolution," she said.

Ms da Silva said the BBC's evidence put China in violation of the arms embargo.

The UN panel of experts on Darfur has said it wants to examine the BBC's evidence.

Homes scorched

The BBC found witnesses who said they saw the first Dong Feng which the BBC tracked down being used with its anti-aircraft gun in an attack in a town called Sirba, in West Darfur, in December.

"When it is shooting or firing there is nowhere for you to move and the sound is just like the sound of the rain. Then 'Bang! Bang! Bang! Bang!'" said Hamaad Abakar Adballa, a witness in the Chadian refugee town of Birak.

The lorry's powerful anti-aircraft gun fired straight into civilian houses. The gun carries high calibre shells that explode on impact, spreading hot shards of metal and causing terrible wounds

Witnesses saw one hut take a direct hit from the gun:

"An intense wave of heat instantly sent all the huts around up in flames," one witness, Risique Bahar, said. "There was a lot of screaming."

In the attack on Sirba one woman was burnt to death, another horribly injured.

Genocide accusation

Sudan's government has been accused by the United States of genocide against Darfur's black Africans.

Prosecutors at the International Criminal Court (ICC) say war crimes by Sudan's Arab-dominated government have included summary executions, rape and torture.

Recently the conflict has deteriorated into more confused fighting, with rebel and militia groups also fighting each other. Two hundred thousand people have been displaced already this year.

Malnutrition rates are set to soar in South Darfur later this year due to insecurity and drought.

Darfur's landscape is spotted with blackened circles representing the hundreds of the villages that were burnt down by government forces and their Janjaweed allies.

Air attacks

In these attacks Darfur's civilians have been hunted not just from the ground, but from the sky.

Most civilians who tell stories of aerial attacks talk about Russian made Antanovs and helicopter gunships.

Many also talk about fighter jets being used, but no-one has ever answered the question of which type of fighter jets these are.

Kaltam Abakar Mohammed, a mother of seven, watched three of her children being blown to pieces as they were attacked by a fighter jet on 19 February in the town of Beybey in Darfur.

The BBC has established that Chinese Fantan fighter jets were flying on missions out of Nyala airport in south Darfur in February.

Panorama acquired satellite photographs of the two fighters at the airport on 18 June 2008, and its investigations indicate these are the only fighter jets that have been based in Darfur this year.

When Kaltam heard the sound of fighting early that morning, she took her children and ran.

"We start running near the well," she said. "We hid behind a big rock. Something that looks like an eagle started coming from over there. It looked like an eagle but it made a funny noise."

When the plane unleashed two bombs Kaltam's five-year-old daughter, Nura, was dismembered from the chest up.

Her eight-year-old son, Adam, was killed instantly, as was her 20-year-old daughter, Amna.

Kaltam's 19-month-old grandson still has shrapnel in his head from the fighter jet bombing. He cries a lot and often calls out for his mother, but she was killed in the attack.

Kaltam's 13-year-old girl, Hawa, cannot grasp what she saw happen that day to her brother and two sisters. She rarely speaks now.

Pilot training

The Chinese Fantan jets are believed to have been delivered to Sudan in 2003 before the current UN arms embargo was imposed on Darfur.

But the BBC has been told by two confidential sources that China is training Fantan fighter pilots.

Sudan imported a number of fighter trainers called K8s two years ago - they are designed to train pilots of fighters like Fantans.

"Clearly this is what they used to train for operations with the Fantans," said Chris Dietrich, a former member of the UN panel on Darfur.

International lawyer Ms da Silva says if China is training Fantan pilots, this represents another Chinese violation of the UN arms embargo.

"The terms of the embargo cover not only just the supply of weapons, military vehicles, paramilitary equipment. It also covers training any technical assistance, so the training of pilots obviously falls within the scope of the embargo."

There are strong economic ties between the China and Sudan.

China buys most of Sudan's oil and believes that what Sudan needs is good business partners, help with development and a solid peace process in Darfur, instead of confrontation and sanctions from the West.

So when China's President Hu Jintao visited Sudan in 2007 he wrote off millions of dollars worth of debt and donated a multi-million pound interest free loan for a new presidential palace to Sudan's President Omar al-Bashir.

In April last year, China's military leaders pledged to strengthen co-operation with Sudan.

Panorama: China's Secret War will be on BBC One at 2030 BST on Monday 14 July 2008.