Three shoppers died and at least 31 others were injured in a stampede when the doors opened at the start of a sale at a Carrefour supermarket in southwestern China on Saturday, state media reported.

Some shoppers slipped as they rushed through a door to the Carrefour in Chongqing city when the store opened at 8:30 a.m., and those that fell were then crushed by other shoppers, the Web sites of the Xinhua News Agency and People's Daily reported.

The People's Daily said shoppers began lining up outside the store at 4 a.m. to buy discounted rapeseed oil used for cooking.

Seven of the 31 injured were in a serious condition, said Xinhua.

Police in Chongqing said they had heard about the accident but declined to independently confirm the media accounts. A doctor at Chongqing's Shapingba District People's Hospital said the emergency room was treating some of the injured but declined to give numbers.

Government officials ordered an investigation.

Telephone calls to government offices in Chongqing, to the Carrefour in Chongqing's Shapingba district and to French-based Carrefour SA's China headquarters in Shanghai rang unanswered.

The accident is the second-known supermarket stampede in recent weeks. Fifteen shoppers were injured in a Shanghai market last month. Both were apparently caused in part by sales of cooking oil, prices of which have soared by more than a third due to inflation in the past year.

As part of promotions to celebrate its 10th anniversary in Chongqing, Carrefour offered 20 percent off 5-liter (1.32 gallon) containers of rapeseed oil, the People's Daily said.