ZHENGZHOU - The death toll from an expressway viaduct collapse in central China's Henan Province on Friday has risen to nine, with the incident caused by an explosion involving a truck carrying fireworks, an initial investigation found.

Seven people died at the scene of the accident, while two others lost their lives on the way to hospital.

Thirteen other people were also injured in the accident, which occurred at 8:52 a.m. on a 30-meter-high viaduct in Mianchi County, Sanmenxia City, about 193 km west of the provincial capital of Zhengzhou.

Of the 13 injured, at least four are in a serious condition, according to rescuers at the scene.

An 80-meter-long section of the southern lane of the viaduct collapsed after the explosion, sources with the rescue headquarters said.

At least 25 vehicles fell from the viaduct, nine of which had been pulled from debris as of 5 p.m., the sources said.

Search and rescue efforts were ongoing at 6:30 p.m.. The first batch of about 200 rescuers, aided with three cranes, were sifting through the debris.

Xinhua reporters saw at least six crushed sedans as well as debris of an oil tanker and a container under the viaduct. The ground was scattered with timber and paper boxes.

Traffic controls have been put in place on the expressway linking Lianyungang City of east China's Jiangsu Province and Horgos, a border port in Xinjiang Uygur Autonomous Region in the country's far west.

Passenger buses from Zhengzhou to Sanmenxia have been suspended.

The Ministry of Public Security has dispatched a team to direct the rescue work and investigate the cause of the accident.

Headed by Huang Ming, vice minister of public security, the team is on its way to the scene, sources with the ministry told Xinhua.

Although the blast was to blame for the collapse, according to the initial investigation, Internet users are suspicious of the quality of the viaduct.

"I just cannot understand how this road was built," said "Wushifeng 521," on the Twitter-like Sina Weibo microblogging service.

"How can fireworks be so powerful? Is the viaduct a piece of shoddy construction work?" asked user "Langren-088."

Accidents involving fireworks are likely to occur around this time of the year in China, as setting them off is a traditional way to celebrate the Spring Festival, or Chinese Lunar New Year, which falls on February 10 this year.

A fireworks blast in north China's Hebei Province on January 18 destroyed a two-story home where the explosives were stored, killing three people and injuring 11 others.