BEIJING, August 4, 2008 (AFP) - Suspected terrorists killed 16 policemen in China's northwest Xinjiang region on Monday, state press reported, the latest in a series of security alerts ahead of the Olympics.

Following are key facts about Xinjiang and the alleged threat there:

GEOGRAPHY: The region's vast 1.6 million square kilometres (618,000 square miles) accounts for a sixth of China's territory and spans into Central Asia. It borders Afghanistan, the former Soviet republics of Tajikistan, Kyrgyzstan and Kazakhstan, as well as Russia to the north and the Pakistan- and Indian-controlled parts of Kashmir to the south.

POPULATION: 20 million, representing 47 ethnic groups. The largest group are the 8.3 million ethnic Uighurs -- a Turkic-speaking central Asian people. But the number of Han Chinese in the region has risen from six percent in 1949 to more than 40 percent now. Critics say it is part of a policy of Han Chinese migration to dilute any nationalist tendencies.

HISTORY: China has long ruled Xinjiang in various degrees and re-established its control there in 1949 by crushing the short-lived state of East Turkestan that had emerged during the earlier Chinese civil war.

Uighurs say they have suffered under Chinese rule. Nationalist sentiment intensified in the 1990s after Soviet troops retreated from Afghanistan and three neighbouring Islamic Soviet republics gained independence.

ALLEGED TERRORIST THREAT: Groups identifying themselves as the East Turkestan Islamic Movement (ETIM) and the Turkestan Islamic Party (TIP) want to establish an independent homeland for ethnic Uighurs. Some experts believe the two groups are one. Washington and Beijing say ETIM militants have received training and funding from Al-Qaeda, although some analysts dispute that.

PRE-OLYMPIC SECURITY INCIDENTS IN XINJIANG AND ELSEWHERE IN CHINA:

August 4 -- China says suspected terrrorists kill 16 policemen at a police station in Kashgar, western Xinjiang.

July 21 -- Three buses bombed in Kunming, southwest China's Yunnan province, killing two people. TIP claimed responsibility. China says TIP not responsible, but gives no details.

July 10 -- China announces it has detained 82 'suspected terrorists' from five groups in Xinjiang this year who had 'plotted sabotage against the Beijing Olympics'.

July 8 -- Chinese authorities say police killed five Muslims who were planning a 'holy war' in Urumqi, Xinjiang's capital

May 5 -- Three killed in bus bombing in Shanghai. TIP claimed responsibility.