India is struggling to unravel the mystery surrounding a United Arab Emirates Air Force aircraft detained in Calcutta since Sunday for carrying undeclared weapons — including at least one missile — bound for China.

The discovery has raised eyebrows, as the UAE buys most of its weapons from the United States and European Union, which impose strict controls on arms transfers to China.

The most controversial theory is that the weapons include high-tech equipment that China would like to examine or copy.

Last year the UAE purchased 14 Maverick air-to-ground missiles from the United States and also signed a contract to buy US Patriot air defence missiles.

The UAE has refused to comment on the matter, fuelling suspicions that the three boxes of weapons found on the Hercules C130 transport plane were supposed to be secret. The aircraft was refuelling en route from Abu Dhabi to the northern Chinese city of Xianyang — a big arms production centre.

The nine crew members have been detained at a hotel and questioned, while the aircraft has been held at Calcutta’s airport, according to Indian officials.

“The crew didn’t mention about the arms and ammunitions content in the initial declaration,” one customs official in Calcutta told The Times.

“The pilot told us that we would be opening the cargo at our own peril. So, the entire cargo was sealed.”

The crew also has yet to explain why they were taking such an indirect route to China, rather than flying over Pakistan.

India’s Foreign Ministry issued a statement saying the pilot, Major Ibrahim Alshamsei, had admitted carrying arms, ammunition and explosives but had failed to declare the items in his initial application.

“The UAE authorities... have since formally regretted the omission in clearly indicating items carried by the aircraft and have described it as a ‘technical error’,” the statement said.

The statement said that India would facilitate the aircraft’s “early release” although it did not specify whether the aircraft would continue to China or return to Abu Dhabi.

Analysts said the weapons could be Chinese samples being returned to Xianyang after tests in the UAE. They could also have been smuggled out of the UAE by individuals without the knowledge of the government there.

Analysts say the UAE is unlikely to risk transferring US missile technology to China for fear of jeopardising future US arms sales.

However, it would face less fallout from transferring one of the MICA missiles it bought from France to use on its Mirage fighter jets.

China would be interested in procuring a MICA as France has also sold them to Taiwan, China’s arch rival, for use on its Mirages.

France cannot sell directly to China because of an EU arms embargo imposed after the killing of protesters in Tiananmen Square in 1989.

“I would imagine the Chinese would be very interested in looking at the UAE’s French missiles,” said Siemon Wezeman of the Stockholm International Peace Research Institute’s Arms Transfer

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