The violence in Xinjiang has not occurred completely out of the blue.

Despite economic development, life for some Uighurs is said to be harder

Its root cause is ethnic tension between the Turkic Muslim Uighurs and the Han Chinese. It can be traced back for decades, and even to the conquest of what is now called Xinjiang by the Manchu Qing dynasty in the 18th Century.

In the 1940s there was an independent Eastern Turkestan Republic in part of Xinjiang, and many Uighurs feel that this is their birthright.

Instead, they became part of the People's Republic of China in 1949, and Xinjiang was declared one of China's autonomous regions, in deference to the fact that the majority of the population at the time was Uighur.

This autonomy is not genuine, and - although Xinjiang today has a Uighur governor - the person who wields real power is the regional secretary general of the Chinese Communist Party, Wang Lequan, who is a Han Chinese.

Inward migration

Under the rule of the Communist Party, there has been considerable economic development, but life has been made more difficult for the Uighurs over the past 20-30 years by the migration of many young and technically-qualified Han Chinese from the eastern provinces.

These new migrants are far more proficient in the Chinese language than all but a few Uighurs, and tend to be appointed to the best jobs.

Not surprisingly, this has created deep-seated resentment among the Uighurs, who view the migration of Han into Xinjiang as a plot by the government to dilute them, undermine their culture and prevent any serious resistance to Beijing's control.

More recently, young Uighurs have been encouraged to leave Xinjiang to find work in the rest of China, a process that had already been under way informally for some years.

There was particular concern at government pressure to encourage young Uighur women to move to other parts of China in search of employment - stoking fears they might end up working in bars or nightclubs or even in prostitution, without the protection of family or community.

Islam is an integral part of the life and the identity of the Uighurs of Xinjiang, and one of their major grievances against the Chinese government is the level of restriction imposed on their religious practices.

There are far fewer mosques in Xinjiang than there were before 1949, and they are subject to severe restrictions.

Children under the age of 18 are not permitted to worship in the mosques, and neither are officials of the Communist Party or the government.

Madrasas - religious schools - are also strictly controlled.

Other Islamic institutions that were once a central part of religious life in Xinjiang have been banned, including many of the Sufi brotherhoods, which are based at the tombs of their founders and provided many welfare and other services to their members.

All religions in China are subject to control by the State Administration for Religious Affairs, but the restrictions on Islam among the Uighurs are far harsher than against most other groups, including the Hui who are also Muslims but are Chinese speakers.

This severity is a result of the association between Muslim groups and the independence movement in Xinjiang, a movement that is absolute anathema to Beijing.

There are groups within Xinjiang that support the idea of independence, but they are not allowed to do so openly because "splitting the motherland" is viewed as treason.

During the 1990s - after the collapse of the Soviet Union and the emergence of independent Muslim states in Central Asia - there was an upsurge in open support for these "separatist" groups, culminating in huge demonstrations in the city of Ghulja in 1995 and 1997.

Beijing suppressed those demonstrations with considerable force, and activists were either forced out of Xinjiang into Central Asia and as far away as Pakistan or were obliged to go underground.

'Climate of fear'

Severe repression since the launch of a "Strike Hard" campaign in 1996 has included harsher controls on religious activity, restrictions on movement, the denial of passports and the detention of individuals suspected of support for separatists and members of their families.

This has created a climate of fear and a great deal of resentment towards the authorities and the Han Chinese.

It is surprising that this resentment has not erupted into public anger and demonstrations before now, but that is a measure of the tightness of control that Beijing has been able to exercise over Xinjiang.

There are a number of emigre Uighur organisations in Europe and the United States; in most cases they advocate genuine autonomy for the region.

In the past, Beijing has also blamed an Eastern Turkestan Islamic Movement for causing unrest, although there is no evidence that this ever existed in Xinjiang.

The authorities in Beijing are unable to accept that their own policies in Xinjiang might be the cause of the conflict, and seek to blame outsiders for inciting the violence - as they do in the case of the Dalai Lama and Tibet.

Even if Uighur emigre organisations wished to provoke unrest, it would be difficult for them to do so and there are, in any case, sufficient local reasons for unrest without the need for external intervention.

Michael Dillon is the former director of the Centre for Contemporary Chinese Studies at the University of Durham. He is also the author of a book entitled Xinjiang: China's Muslim Far Northwest.