BEIJING (AFP) - - China's Olympic run-up stuttered again Monday as it reported that terrorists killed 16 policemen in the nation's far northwest, while pollution returned to fill athletes' lungs in Beijing.

The attack in the Muslim-populated Xinjiang region raised the security temperature ahead of the Games, which begin on Friday, as authorities had repeatedly warned that militants there were planning to sabotage the Games.

It also follows deadly bomb blasts in the southwestern city of Kunming last month and in Shanghai in May, killing a total of five people, which a Muslim militant group with ties to Xinjiang claimed responsibility for.

The Chinese organisers of the Games said they were checking for any link between Monday's attack and the Olympics, but immediately sought to reassure the world about security arrangements for the event.

"We have strengthened security work in all Olympic venues and in the Olympic village. We are well-prepared in security for the upcoming Games," Beijing Olympic organising committee spokesman Sun Weide said.

According to the official version of the attack published in the state-run media, two assailants in Xinjiang's famed Silk Road city of Kashgar killed 16 policemen and injured another 16.

The pair drove a truck at the police officers who were jogging near their barracks, the Xinhua news agency said.

After the truck hit a roadside pole, the two got off and threw home-made explosives at the barracks, then moved in to hack at police officers with knives, Xinhua reported, adding that both attackers were arrested.

Kashgar is 4,000 kilometres (2,500 miles) from Beijing, close to the Tajikistan border.

Xinhua did not identify who the terrorists may be affiliated to, but China has said previously that Muslim groups seeking independence for Xinjiang and the creation of "East Turkestan" were a major security threat.

Xinjiang, a vast area that borders Central Asia, has about 8.3 million ethnic Muslim Uighurs , and many are unhappy with what they say has been decades of repressive Communist Chinese rule.

The East Turkestan Islamic Movement (ETIM), which reportedly operates inside Xinjiang and in neighbouring Afghanistan, is listed by China, the United States and the United Nations as a terrorist organisation.

However exiled Uighur dissidents and some human rights groups say China's claims that the ETIM is a major threat are exaggerated.

Last month, a group calling itself the Turkestan Islamic Party (TIP) claimed credit for the deadly bus blasts in Shanghai and Kunming. Some experts believe TIP is part of ETIM.

After raising the alarm about Olympic terrorist attacks, China denied the TIP carried out those attacks, but said nothing more as to who may be responsible.

One of the other problems for China in the Olympics run-up has been Beijing's notorious pollution, which International Olympic Committee president Jacques Rogge said last year could lead to some endurance events being postponed.

After relatively clear skies over the weekend that led Chinese officials to trumpet the success of drastic anti-pollution measures, a familiar heavy smog permeated the city on Monday.

One million of the city's 3.3 million cars were taken off the roads from July 20, and more than 100 heavily polluting factories and building sites were closed down.

Chinese authorities have said they could take further measures if air quality remained poor, and officials were quoted in the state press on Monday as saying those emergency plans may kick in soon.

"Should environmental departments foresee serious air pollution during the Olympics, Beijing and neighbouring areas will temporarily close more factories and pull more cars off the road," Fan Yuansheng, director of pollution control at the Ministry of Environmental Protection, was quoted as saying.