The incident highlights the continuing trend of drug trafficking in Asia, with the primary production centre shifting from the Golden Triangle (Myanmar, Thailand and Laos) to the Golden Crescent (Afghanistan, Pakistan and Iran) and the mainland turning from the biggest transition point for these illicit goods to the primary market where they are sold.

'In the 1990s, the most important route for drug trafficking was from Myanmar into Yunnan or Guizhou and then to Guangdong or Fujian . From there, heroin would be smuggled to Hong Kong, where it could fetch top price,' said a regional government staff worker familiar with anti-narcotics work.

'This has since changed. Since 2004, most heroin has been flowing in from Afghanistan. It is often sold in big mainland cities - not just Guangzhou, Shenzhen or Shanghai, but also Urumqi , Chongqing or Chengdu ,' he said.

As mainland society becomes more affluent, the demand for high-grade heroin is rising. While heroin is losing out to other recreational drugs in other parts of the world, it remains the most used illicit drug on the mainland.

According to government figures, about 80 per cent of the drug addicts on the mainland use heroin.

'Back in the 1990s, or even the early 2000s, the price for a kilogram of grade-four heroin in Hong Kong was several times higher than that sold on the mainland. Now the gap is much narrower. The profit margin [in Shenzhen] is probably higher for drug dealers than in Hong Kong - once you factor in risk and trafficking expenses,' the staffer said.

This, coupled with the sudden demise of the opium trade in the Golden Triangle, increased the demand for high-grade heroin on the mainland. The United Nations report on global narcotics in 2007 stated that opium production in the Golden Triangle had dropped from 33 per cent of the world's total in 1998 to just 5 per cent in 2007. The Golden Crescent, in particular Afghanistan, has become the undisputed primary production centre for heroin.

While the US remains the world's biggest market for heroin, the mainland market is growing quickly. In the Xinjiang regional capital of Urumqi alone, police estimate the amount of heroin consumed by addicts rose from one tonne in 2000 to seven tonnes in 2007, a mainland media report said.

The 2009 Anti Narcotics Report by the Ministry of Public Security stated: 'Opium production areas in the Golden Triangle are shrinking fast. The amount of heroin trafficking from northern Myanmar into our country is obviously lower. This has resulted in a significant shortage of supply in the heroin consumer market [in China].'

This has created a market for heroin from the Golden Crescent.

Known by mainland addicts as 'Afghanistan Tobacco', top-grade heroin from the Golden Crescent sells for 550 to 600 yuan (HK$625 to HK$680) a gram in major cities such as Urumqi. The street prices in Guangzhou or Shenzhen could be even higher. The price in Hong Kong is about HK$1,000.

As border patrols and anti-drug campaigns have been stepped up in Xinjiang and other western provinces, drug dealers are starting to use the maritime route to smuggle the illicit goods into the mainland.

A Shenzhen official told Beijing-friendly newspaper Ta Kung Pao that the four arrested foreigners could face the 'maximum penalty' if convicted.

Shenzhen customs officials said they received a tip-off about the heroin smuggling on August 20, and seized the drug hidden in cargo from Pakistan on September 2.

A total of 289 small bags of heroin weighing 0.5kg each were found hidden in 6,720 bales of cotton. Police later arrested a foreigner and his mainland accomplice as they were waiting outside a Guangzhou warehouse for the goods to be delivered. Another seven people, including the manager of a Shenzhen shipping company, were arrested following the investigation.

Police believe the nine worked for a multinational drug-trafficking group. Shenzhen officials said a Pakistani gang leader was also arrested in Pakistan.

The case was disclosed just two days after Akmal Shaikh, a 53-year-old Briton, was executed in Urumqi for smuggling 4kg of heroin into China in 2007. The execution went ahead despite intensive international appeals on the grounds that he was mentally unstable. The incident triggered a bitter row between Beijing and London, and condemnation of China from the European Union.

Beijing has imposed harsh penalties in an attempt to contain the spread of the drug trade. It has not shown lenience to foreigners, who would normally be spared from capital punishment. In June, Dongguan Intermediate People's Court sentenced a Nigerian and two mainland drug traffickers to death for selling nearly 60kg of heroin.