Three people have been killed and 79 others were injured in an explosion that rocked a railway station in China's restive far-western region of Xinjiang as President Xi Jinping wrapped up a four-day visit to the area, state media reported.



State news agency Xinhua said that four of the people had been seriously injured in the blast which hit outside an exit at the station in Urumqi on Wednesday evening, Reuters news agency said. Assailants also used knives, and detonated explosive devices, the news service said.

Xinhua called the explosion a "violent terrorist attack", the AFP news agency reported. It was unclear whether Xi was still in the region at the time of the blast.

Train service was suspended for about two hours before it reopened under the watch of armed police, Xinhua reported, according to the AP news agency.

A woman working at a shop near the train station said she heard a loud explosion shortly after 7pm. "The whole area now has been cordoned off by police and military police," the woman told AP.

Photos circulating briefly on Chinese social media sites showed scattered luggage near the station's exit and a heavy presence of armed men. Xinhua said the blast was centred on some luggage left on the ground between the station's exit and a bus stop.



Ethnic tensions have been simmering for years in Xinjiang, the home of the Muslim Uighur minority group. In 2009, a series of riots broke out in Urumqi, leaving nearly 200 people dead, according to official figures. Despite a heavy crackdown, violence has continued in the region and began spreading elsewhere in the country last year.



Rian Thum, a professor at Loyola University in New Orleans, who specialises in Uighur history and issues, said the use of explosives and the location of Wednesday's blast were significant.



"Violence in Xinjiang has previously tended to target security personnel and officials, often carried out with knives or farm tools. A bomb attack targeting civilians, if that is what this is, would mark an alarming deviation from previous patterns of Uighur political violence," he said.



"Of course, we can't yet assume that the attack was intentional, or that it was carried out by Uighurs, though it seems likely that it was both.



Last year, three Uighurs rammed a vehicle into crowds in a suicide attack near the Forbidden City gate in the heart of Beijing, killing themselves and two tourists.



In March, five knife-wielding men and women believed to be Uighurs slashed at crowds indiscriminately at a railway station in southwestern China, killing 29 people. The government has blamed the attack on "secession-seeking terrorists."