URUMQI, China -- Attackers hurled bombs from two sport utility vehicles that plowed through shoppers at a busy street market in China's volatile northwestern Xinjiang region Thursday, killing 31 people and wounding more than 90.

The early morning assault in Urumqi was the bloodiest in a series of violent attacks over recent months that Chinese authorities have blamed on radical separatists from the country's Muslim Uighur minority group.

The Xinjiang region is home to the native Turkic-speaking Uighurs but has seen large inflows from China's ethnic Han majority in recent decades. Uighur activists contend that restrictive and discriminatory policies favoring the Chinese migrants are fueling the bloodshed.

The two vehicles crashed through barriers at 7:50 a.m. and drove right into the crowds while its occupants set off explosives, according to a statement by the Xinjiang regional government. It was unclear how many assailants were in the cars.

The SUVs then crashed head-on and one of them exploded, the official Xinhua News Agency reported. It quoted an eyewitness as saying there were up to a dozen blasts in all and that at one point one of the vehicles stopped because it was blocked by bodies and handcarts.

"I heard four or five explosions. I was very scared. I saw three or four people lying on the ground," said Fang Shaoying, the owner of a small supermarket near the scene of the attack.

Photos from the scene posted to the popular Chinese social-media website Weibo showed at least three people lying in a street with a large fire in the distance giving off huge plumes of smoke. Others, many of them elderly people who frequent the early morning market, were sitting in the roadway in shock, with vegetables, boxes and stools strewn around them.

Hours afterward, armed paramilitary police and SWAT units patrolled streets surrounding the scene of the attack on Gongyuan North Street near central People's Park. News photographers and videographers were told not to take pictures and were forced to delete their images.

Paramilitary troops were deployed all along the street, a hub for grocery stores, restaurants and bars, many of which were closed.

However, traffic had resumed, and there were no signs of the earlier explosion and fire.

The death toll was the highest for violence in Xinjiang since days-long riots in Urumqi in 2009 between Uighurs and Hans left almost 200 people dead, according to an official count. Thursday's attack also was the bloodiest single act of violence in Xinjiang in recent history.

Urumqi was the scene of a bomb attack at a train station late last month that killed three people, including two attackers, and injured 79. Security in the city has been significantly tightened since that attack, which took place as Chinese President Xi Jinping was visiting the region.

In response to Thursday's attack, Xi pledged to "severely punish terrorists and spare no efforts in maintaining stability," Xinhua reported.

Public Security Minister Guo Shengkun, China's top police official, was dispatched to Urumqi as the head of a team to investigate the attack.

Russian President Vladimir Putin, who visited China earlier in the week for an Asian security conference, issued a statement saying he "resolutely condemned the bloody crime." Russia, which also faces violence from Muslim extremists, wishes to strengthen cooperation with Beijing "in fighting all forms of terrorism and extremism," the statement said.

In Washington, the White House also issued a statement denouncing the "despicable and outrageous act of violence against innocent civilians" and noting that "the United States resolutely opposes all forms of terrorism."

Before last month's train station attack, Urumqi had been relatively quiet since the 2009 ethnic riots. The sprawling metropolis' population of more than 3 million people is about three-fourths Han Chinese.

The station attack and other violence have been blamed on Uighur extremists, though information about events in the area, which is about 1,550 miles west of Beijing, is tightly controlled.

In an unprecedented attack 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.

And in March, 29 people were slashed and stabbed to death at a train station in the southern city of Yunnan. The attack was blamed on Uighur extremists bent on waging jihad.

The increasing frequency of attacks shows growing frustration among Uighurs over government policies seen as discriminatory, said Raffaello Pantucci, a terrorism expert at the Royal United Services Institute for Defense and Security Studies in London.

"The issues are not getting resolved, and in some ways are getting worse," Pantucci said. "People are left feeling they have no hope."

Information for this article was contributed by Christopher Bodeen, Aritz Parra and Andy Wong of The Associated Press.

A Section on 05/23/2014