A fireworks-laden truck exploded as it crossed a bridge in central China on Friday, killing 26 people as the structure collapsed and vehicles plummeted to the ground, state-run media reported.

The lorry "suddenly exploded" on the expressway bridge in Henan province, China National Radio said, putting the number of deaths at 26.

An 80-metre long part of the bridge collapsed and six vehicles had been retrieved from the debris, China's official news agency Xinhua said, adding that at least four people died and rescue efforts were underway.

Pictures posted online appeared to show tangled wreckage beneath the bridge, including a red truck lying on its side, and another upturned with its wheels pointing skywards, as rescue workers in orange uniforms scaled the wreckage.

A crowd of people peered at the debris through thick hazy fog as another lorry partially hung off the bridge's mangled edge.

The bridge near the city of Sanmenxia is on the G30 expressway, the longest road in China, which stretches for nearly 4,400 kilometres (2,700 miles) from China's western border with Kazakhstan to the eastern Yellow Sea.

It stands 30 metres above the ground, local media reported.

Many of China's roads are packed as the country gears up for its most important holiday, the Lunar New Year, when hundreds of millions of people travel back to their hometowns in what is billed as the world's largest annual human migration.

The season is also a peak period for fireworks producers, as Chinese traditionally buy the explosives to celebrate the festival, which falls on February 10 this year.

According to police statistics, around 70,000 people die and 300,000 more are injured in road accidents every year in China, state media have reported.

China has seen a number of bridge collapses in recent years, which are often blamed on overloading and shoddy construction. One such incident in northeast China last August killed three people.