China's Olympic run-up stuttered again as 16 policemen were killed in a grenade attack 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 Olympics, the AFP reported.
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, for which a Muslim militant group with ties to Xinjiang claimed responsibility.
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 believe that we are prepared to deal with any kind of security threat and we are confident and capable of hosting a peaceful and safe Games," said Beijing Olympic organising committee spokesman Sun Weide.
Nevertheless, police in Hong Kong, the southern Chinese city hosting the Olympic equestrian events, said they would reassess security there to see if the attack had any implication for the competition.
According to the official version of the attack published in 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 in the northwestern outskirts of the city, the Xinhua news agency said.
After the truck hit a roadside pole, the two got out 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 with, but China has previously said that Muslim groups seeking independence for Xinjiang and the creation of " East Turkestan" were a major security threat.
The exiled leader of China's Uighur Muslims condemned the reported killing of 16 policemen.
"We condemn all acts of violence," Rebiya Kadeer said in Washington, where she has been living in exile since 2005 after spending six years in a Beijing prison. "The Uighur people do not support acts that engender bloodshed."
Kadeer's Uighur American Association said it was seeking independent accounts of the incident and urged the international community "to view Chinese government accounts regarding Uighur terrorist acts with caution, as government authorities consistently fail to provide evidence to back up their claims."
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 were 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.
Another problem facing 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 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 in efforts to clear the air ahead of the Games.
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.
Nevertheless, when asked about the return of the clearly visible pollution, Olympic organising committee spokesman Sun insisted the quality of the air was good on Monday and that there would be no problem for athletes.