Hong Kong (CNN) -- A truck carrying fireworks on an expressway bridge in central China exploded Friday, causing part of the bridge to collapse and sending dozens of vehicles plunging off the edge, authorities said.

The exact number of casualties wasn't immediately clear. Highway police in Henan province, where the explosion happened, said on their official microblog account that at least 11 people had been killed.

But that post was later deleted and replaced with one that gave the lower total being reported by Xinhua, the official state news agency. By late Friday afternoon, Xinhua was citing local authorities as saying that eight people had been killed.

The state-run China National Radio had earlier reported on its website that as many as 26 people had died in the disaster. It didn't say where it got the information from.

Authorities have closed the expressway while search and rescue efforts are under way, Xinhua reported, and 13 injured people have been retrieved from the wreckage so far.

At least 25 vehicles are believed to have fallen off the bridge to the ground about 30 meters (100 feet) below, Xinhua said.

State broadcaster CCTV carried footage of a yawning gap in one part of the bridge, with mounds of debris, including rubble and parts of vehicles, spread out below

Rescue workers in bright orange overalls clambered over upturned trucks, looking for survivors.

Xinhua said an 80-meter (260-foot) stretch of the bridge had collapsed after the explosion, which occurred at 8:52 a.m. local time in Mianchi County.

China's fireworks tradition

Fireworks are an enduring element of celebrations of the Lunar New Year in China, one of the country's most important holidays that takes place this month. But they have been at the root of accidents in the past.

In 2009, fireworks set off a huge fire that gutted a brand-new hotel in central Beijing, briefly prompting calls for the return of a ban put in place at the height of Chairman Mao Zedong's rule in the 1970s.

According to local folklore, fireworks drive away monsters and evil spirits. But under Mao they were prohibited, ostensibly on the basis that they were "bourgeois" and a "waste of money."

Beijing authorities on Friday urged residents to set off fewer fireworks during this year's Lunar New Year celebrations to avoid exacerbating the thick pollution that has cloaked the capital for much of the past month, Xinhua reported.

CNN's CY Xu in Beijing contributed to this report.