NANJING - Thirteen people were confirmed dead and 21 people injured after a collision between a tour bus and a truck on Sunday morning, Jiangsu provincial government confirmed Sunday.

Fire fighters carry an injured passenger after a collision between a truck and bus killed 13 people and left many injured in the Changshu section on an expressway in E?China's Jiangsu Province, April 22, 2012.[Photo/Xinhua]?

The accident occurred at 9:25 am after a tour bus from Shanghai drove through the barriers in the middle of the expressway and collided with a heavy truck coming in the opposite direction in the Changshu section of the Yanjiang Expressway in Jiangsu Province.

Both vehicles were overturned at the scene, the government spokesman said.

According to official reports, 33 passengers were aboard the bus when the accident happened. Six bus passengers and the driver of the truck were killed instantly, and six other people died after rescue efforts failed.

Among the 21 injured, four are still in critical condition.

The tour bus left Shanghai People's Square at 8:00 am and was heading to Shanghu in Changshu, for a peony show.

The cause of the accident is under investigation.