“I can confirm that 53 people have died in the accident,” Harry Kalaba, an official in the vice president’s office, told AFP from the crash site 100 kilometres (60 miles) north of Lusaka.
“Some bodies are still trapped under the truck but we called for a crane to lift the truck.”
The bus belonged to the Zambia Postal Service and was heading from the Copperbelt mining province to Lusaka.
Photos posted on social media sites appeared to show bloodied and lifeless bodies lying next to the mangled wreckage.
Local media reported as many as 70 passengers were on the bus, but Kalaba could not confirm the figure.