The blast, which destroyed an 80m (262ft) section of road, took place on the G30 expressway in Henan province.
China National Radio had put the number of dead at 26 but the report appeared to have been removed from websites.
Several vehicles were reported to have fallen to the ground 30m below.
Witnesses said some cars were thrown over the edge by the force of the blast, the cause of which was not immediately clear.
Six vehicles were retrieved from the wreckage, Xinhua said, while search and rescue efforts were continuing at the site. Eight people had been injured, the agency said.
Footage from state broadcaster CCTV showed rescuers digging through rubble. The channel said 10 trucks had been found under the bridge.
Images posted on microblogging sites showed wrecked trucks and debris, as well as a collapsed section of highway.
One photo showed a truck perched precariously at the broken edge of the carriageway.
The G30 is the longest road in China, running for 4,400km (2,700 miles) from the coast of eastern Jiangsu province to Urumqi, close to the Kazakhstan border.
Fireworks are traditionally set off in China to celebrate the Lunar New Year, which is just over a week away. There are frequently reports of accidents as they are being made, stored or transported.
The BBC's Jon Sudworth in Beijing says there has been much debate over whether the use of fireworks should be reduced, in part to help curb pollution.
Official statistics show that in 2011, more than 60,000 people were killed on roads in China. The deaths are often blamed on inadequate training, poor quality vehicles or on drivers ignoring safety instructions.
Earlier this month, China halted plans to introduce tougher penalties for traffic offences, following a public outcry.
The rules imposed penalty points for motorists who drove through amber traffic lights, with a driving ban after two offences, as well as increasing penalties for speeding, drinking and driving and using a mobile phone while behind the wheel.
But there were widespread complaints that drivers stopping abruptly at lights were ultimately causing more accidents.
The government said it was reconsidering its traffic policies.

It is billed as one of the biggest events of its kind in Europe and  last year attracted more than 30,000 visitors.
This year, organisers invited people to submit photos for the People's Gallery based on the theme chaos and calm.
The best 76 images are to be exhibited at the show - here is a selection.
A truck carrying fireworks has exploded on an elevated highway in central China, killing at least five people and causing part of the road to collapse, state media report.
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Amateur and professional photographers from across the UK were invited to submit pictures for the forthcoming Photography Show at the NEC near Birmingham.