CLX9877 flight from Shanghai to Luxembourg - Flightradar24.

A day after Israel attacked Iran on Friday, a cargo plane took off from China. The next day, a second plane departed from a coastal city. Then on Monday, yet another departed, this time from Shanghai – three flights in three days.

Data showed that on each flight, the plane flew westward along northern China, crossing into Kazakhstan, then south into Uzbekistan and Turkmenistan – and then falling off the radar as it neared Iran.

To add to the mystery, flight plans indicated a final destination of Luxembourg, but the aircraft appeared to have never flown near European skies.

Concern has erupted in response over what might have been sent from China in the direction of Iran as Tehran’s war with Israel rages on.

The worry is real – aviation experts have noted that the type of plane used, Boeing 747 freighters, are commonly used for transporting military equipment and weapons, and hired to fly government contract orders.

Credit: Flightradar24

“These cargos cannot but generate a lot of interest because of the expectation that China might do something to help Iran,” said Andrea Ghiselli, a lecturer at the University of Exeter who specialises in China’s relations with the Middle East and North Africa.

China and Iran are strategic partners, aligned primarily in their opposition to the US-led world order, and in favour of a new “multi-polar” phase in global diplomacy.

Iran, too, is one of China’s key energy suppliers, sending as many as two million barrels of oil a day – so it’s no surprise that Beijing might be looking for ways to support and stabilise the Islamic Republic.

“The collapse of the current regime would be a significant blow and would generate a lot of instability in the Middle East, ultimately undermining Chinese economic and energy interests,” said Mr Ghiselli.

“Moreover, in Iran there are probably many that are expecting some kind of help from China.”

China has a history of supplying Iran despite international criticism – for example sending thousands of tons of ballistic missile materials that could be used in Iran’s development of nuclear weapons.

Still, in this key moment, experts say Beijing is likely approaching with caution.

Getting involved directly in the Iran-Israel conflict could torpedo any change China has to stabilise its relations with the US, Israel’s strongest ally. Beijing is still reeling from a high-stakes trade war with Washington.

“The presence of Chinese military hardware would make that impossible, especially as there are already some that are pushing for the US to join the war also to contain China by attacking Iran,” said Mr Ghiselli.

While “the likelihood remains low” of China overtly sending defence materials to Tehran, the possibility “should not be dismissed and must be closely monitored”, said Tuvia Gering, a China and Middle East specialist at Israel’s Institute of National Security Studies.

Unless independent inspections are carried out, it is not possible to know exactly what the cargo planes were carrying. In later flights, some of the aircraft appear to take off from around the same area along the Turkmenistan-Iran border, and go toward Luxembourg, according to publicly available flight data.

Cargolux, the Luxembourg-based company that operated the planes, said its flights did not utilise Iranian airspace, but the firm did not respond to questions about what they were carrying.

Cargo manifests are not considered a matter of public record, and though any dangerous goods or special loads have to be declared to the operator and handling agents, information provided could be inaccurate or misleading.

China has tried before to send weapons disguised as commercial goods, labelling drone components as wind turbine parts, according to shipments intercepted by European authorities.

A Telegraph investigation last year found that China tried to send $1 billion (£738 million) of drones to Libya, hidden behind a web of shell firms in the UK, Tunisia and Egypt, in exchange for crude oil.